Re: Merge-base alias for git vc-diff

[Juri Linkov]

>> The reason I proposed separate commands is because I can't find
>> a good solution how to combine this with the existing commands.
>
> You'd have to change the notion of "revision" in vc-git.el to allow
> a leading or trailing "...": remember, this is specific to Git so it can
> only belong in vc-git.el.
>
> It also means that revision completion in C-x v ~ will also allow those
> "..." even tho they don't make any sense there.

After trying to implement this with

diff --git a/lisp/vc/vc-git.el b/lisp/vc/vc-git.el
index f9ea4def15..8f8e151549 100644
--- a/lisp/vc/vc-git.el
+++ b/lisp/vc/vc-git.el
@@ -1247,4 +1249,10 @@ vc-git-diff
       (setq command "diff-index")
       (unless rev1 (setq rev1 "HEAD")))
+    (when (string-match "\\`\\(.*\\)\\.\\.\\.\\'" rev1)
+      (setq rev1 (string-trim-right (or (vc-git--run-command-string
+                                         nil "merge-base"
+                                         (match-string 1 rev1)
+                                         rev2)
+                                        (match-string 1 rev1)))))
     (if vc-git-diff-switches
         (apply #'vc-git-command (or buffer "*vc-diff*")

and using with `C-u C-x v D master... RET branch RET'
exposed an unexpected problem: vc-diff-internal sets
diff-vc-revisions to the list ("master..." "branch")

But diff-mode fails to use diff-vc-revisions for such features as
jumping to the corresponding source line with diff-goto-source that
signals an error in vc-git-find-revision:

  (error "Failed (status 128): git --no-pager cat-file blob 
master...:lisp/vc/vc-git.el .")

and leaves the buffer with an error message in it:

  fatal: Not a valid object name master...:lisp/vc/vc-git.el

One way to resolve this is to interpret "master...branch" as a syntax
to denote the merge base, then set diff-vc-revisions instead of

  ("master..." "branch")

to

  ("master...branch" "branch")

This means that this feature should be used only this way:

  C-u C-x v D master...branch RET branch RET