bug#31754: 25.1; etags output lacks info for making xref more useful

[Gauthier Östervall]

Given the following code:

---8<---
int my_identifier(int a) {
    return 2 * a;
}

int my_identifier(int a, int b) {
    return a + b;
}
---8<---

and a TAGS file generated with etags (no options).

Running xref-find-definitions (M-.) opens a suggestion buffer with this content:

---8<---
/home/gauthier/tmp/xref.c
1: int my_identifier(
5: int my_identifier(
---8<---

This could be more helpful, the suggestions are not distinguishable. I
suggest including at least the whole source row.

The example source is only a minimal example, obviously not a real problem in
C. This disturbs me when I have multiple identifier definitions.

---

In GNU Emacs 25.1.1 (x86_64-pc-linux-gnu, GTK+ Version 3.22.11)
 of 2017-09-15, modified by Debian built on trouble
Windowing system distributor 'The X.Org Foundation', version 11.0.11902000
System Description: Debian GNU/Linux 9.4 (stretch)

Configured using:
 'configure --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --with-pop=yes
 
--enable-locallisppath=/etc/emacs25:/etc/emacs:/usr/local/share/emacs/25.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/25.1/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --with-pop=yes
 
--enable-locallisppath=/etc/emacs25:/etc/emacs:/usr/local/share/emacs/25.1/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/25.1/site-lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --with-x=yes --with-x-toolkit=gtk3
 --with-toolkit-scroll-bars 'CFLAGS=-g -O2
 -fdebug-prefix-map=/build/emacs25-wN2qS3/emacs25-25.1+1=.
-fstack-protector-strong
 -Wformat -Werror=format-security -Wall' 'CPPFLAGS=-Wdate-time
 -D_FORTIFY_SOURCE=2' LDFLAGS=-Wl,-z,relro'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11

Important settings:
  value of $LANG: en_US.utf8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: robot mode

Minor modes in effect:
  magit-auto-revert-mode: t
  auto-revert-mode: t
  dumb-jump-mode: t
  flx-ido-mode: t
  global-git-commit-mode: t
  async-bytecomp-package-mode: t
  shell-dirtrack-mode: t
  diff-auto-refine-mode: t
  ido-everywhere: t
  global-cwarn-mode: t
  dtrt-indent-mode: t
  show-paren-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: (only)

Recent messages:
Type C-c C-c to finish, or C-c C-k to cancel
(No changes need to be saved)
Saving file /home/gauthier/.emacs.d/whsettings...
Wrote /home/gauthier/.emacs.d/whsettings
File TAGS is large (11.7M), really open? (y or n) y
Note: indent-tabs-mode adjusted to nil
Mark set [2 times]
Saving file /home/gauthier/.emacs.d/whsettings...
Wrote /home/gauthier/.emacs.d/whsettings
Mark saved where search started

Load-path shadows:
/home/gauthier/.emacs.d/elpa/git-commit-20171007.346/git-commit hides
~/.emacs.d/includes/git-commit
/usr/share/emacs/25.1/site-lisp/debian-startup hides
/usr/share/emacs/site-lisp/debian-startup
/usr/share/emacs25/site-lisp/cmake-data/cmake-mode hides
/usr/share/emacs/site-lisp/cmake-mode
~/.emacs.d/includes/ruby-mode hides
/usr/share/emacs/25.1/lisp/progmodes/ruby-mode

Features:
(shadow sort mail-extr emacsbug sendmail conf-mode flyspell ispell
magit-obsolete magit-blame magit-stash magit-bisect magit-remote
magit-commit epa magit-sequence magit-notes magit-worktree magit-branch
magit-files magit-refs magit-status subr-x magit esh-var esh-io esh-cmd
esh-opt esh-ext esh-proc esh-arg esh-groups eshell esh-module esh-mode
esh-util dired-x magit-repos magit-apply magit-wip magit-log magit-diff
smerge-mode magit-core magit-autorevert autorevert filenotify
magit-process magit-margin magit-mode magit-git magit-section
magit-popup disp-table blank-mode rect misearch multi-isearch projectile
grep ibuf-ext vc-bzr vc-hg haskell-doc inf-haskell haskell-decl-scan
imenu haskell haskell-completions haskell-load haskell-commands
highlight-uses-mode haskell-modules haskell-sandbox
haskell-navigate-imports haskell-repl haskell-collapse hideshow
haskell-debug haskell-interactive-mode haskell-presentation-mode
haskell-compile haskell-hoogle haskell-process haskell-session url-util
url-parse auth-source url-vars json map haskell-mode haskell-cabal
haskell-utils haskell-font-lock haskell-indentation haskell-string
haskell-sort-imports haskell-lexeme rx haskell-align-imports
haskell-compat haskell-complete-module haskell-ghc-support noutline
outline flymake dabbrev haskell-customize dumb-jump popup f etags xref
project eieio eieio-core fill-column-indicator flx-ido flx
google-c-style visual defaultcontent buffer-move windmove git-commit
with-editor async-bytecomp async shell pcomplete server magit-utils
vc-git diff-mode crm log-edit easy-mmode message format-spec rfc822 mml
mml-sec password-cache epg gnus-util mm-decode mm-bodies mm-encode
mail-parse rfc2231 rfc2047 rfc2045 ietf-drums mm-util help-fns
mail-prsvr mailabbrev mail-utils gmm-utils mailheader pcvs-util add-log
highlight-symbol thingatpt copy ws-trim pymacs python-mode info-look
darkroom-mode frame-local-vars w32-fullscreen gas-mode revbufs ido
ibuffer ag vc-svn cl-seq derived compile comint ansi-color ring
find-dired s dash dired column-marker bisque-darker-theme
smart-mode-line cl-macs cl cwarn cc-mode cc-fonts cc-guess cc-menus
cc-cmds cc-styles cc-align cc-engine cc-vars cc-defs hl-line dtrt-indent
cus-start cus-load advice paren finder-inf edmacro kmacro
highlight-symbol-autoloads rainbow-mode-autoloads info package
epg-config seq byte-opt gv bytecomp byte-compile cl-extra help-mode
easymenu cconv cl-loaddefs pcase cl-lib time-date mule-util tooltip
eldoc electric uniquify ediff-hook vc-hooks lisp-float-type mwheel x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list newcomment elisp-mode lisp-mode prog-mode register page
menu-bar rfn-eshadow timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core frame cl-generic cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms
cp51932 hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese charscript case-table epa-hook jka-cmpr-hook help
simple abbrev minibuffer cl-preloaded nadvice loaddefs button faces
cus-face macroexp files text-properties overlay sha1 md5 base64 format
env code-pages mule custom widget hashtable-print-readable backquote
dbusbind inotify dynamic-setting system-font-setting font-render-setting
move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 885150 128230)
 (symbols 48 39371 0)
 (miscs 40 951 1965)
 (strings 32 83523 14685)
 (string-bytes 1 2857273)
 (vectors 16 61900)
 (vector-slots 8 2683541 97025)
 (floats 8 8004 1127)
 (intervals 56 6166 738)
 (buffers 976 33))