bug#33950: 27.0.50; Merge-base alias for git vc-diff

[Juri Linkov]

Following https://lists.gnu.org/archive/html/emacs-devel/2018-12/msg00446.html
here's the patch that implement this feature:

diff --git a/lisp/vc/vc-hooks.el b/lisp/vc/vc-hooks.el
index 84e11f2e01..f32e536981 100644
--- a/lisp/vc/vc-hooks.el
+++ b/lisp/vc/vc-hooks.el
@@ -890,6 +890,8 @@ vc-prefix-map
     (define-key map "L" 'vc-print-root-log)
     (define-key map "I" 'vc-log-incoming)
     (define-key map "O" 'vc-log-outgoing)
+    (define-key map "ML" 'vc-log-mergebase)
+    (define-key map "MD" 'vc-diff-mergebase)
     (define-key map "m" 'vc-merge)
     (define-key map "r" 'vc-retrieve-tag)
     (define-key map "s" 'vc-create-tag)
diff --git a/lisp/vc/vc.el b/lisp/vc/vc.el
index 48b7c98dfa..ccd03c4427 100644
--- a/lisp/vc/vc.el
+++ b/lisp/vc/vc.el
@@ -429,6 +429,10 @@
 ;; - region-history-mode ()
 ;;
 ;;   Major mode to use for the output of `region-history'.
+;;
+;; - mergebase (rev1 &optional rev2)
+;;
+;;   Return the common ancestor between REV1 and REV2 revisions.
 
 ;; TAG SYSTEM
 ;;
@@ -1849,6 +1853,30 @@ vc-root-version-diff
        t (list backend (list rootdir)) rev1 rev2
        (called-interactively-p 'interactive)))))
 
+
+;;;###autoload
+(defun vc-diff-mergebase (_files rev1 rev2)
+  "Report diffs between the merge base of REV1 and REV2 revisions.
+The merge base is a common ancestor between REV1 and REV2 revisions."
+  (interactive (vc-diff-build-argument-list-internal))
+  (when (and (not rev1) rev2)
+    (error "Not a valid revision range"))
+  (let ((backend (vc-deduce-backend))
+        (default-directory default-directory)
+        rootdir)
+    (if backend
+        (setq rootdir (vc-call-backend backend 'root default-directory))
+      (setq rootdir (read-directory-name "Directory for VC root-diff: "))
+      (setq backend (vc-responsible-backend rootdir))
+      (if backend
+          (setq default-directory rootdir)
+        (error "Directory is not version controlled")))
+    (let ((default-directory rootdir)
+          (rev1 (vc-call-backend backend 'mergebase rev1 rev2)))
+      (vc-diff-internal
+       t (list backend (list rootdir)) rev1 rev2
+       (called-interactively-p 'interactive)))))
+
 ;;;###autoload
 (defun vc-diff (&optional historic not-urgent)
   "Display diffs between file revisions.
@@ -2491,6 +2519,25 @@ vc-log-outgoing
     (vc-incoming-outgoing-internal backend (or remote-location "")
                                    "*vc-outgoing*" 'log-outgoing)))
 
+;;;###autoload
+(defun vc-log-mergebase (_files rev1 rev2)
+  "Show a log of changes between the merge base of REV1 and REV2 revisions.
+The merge base is a common ancestor between REV1 and REV2 revisions."
+  (interactive (vc-diff-build-argument-list-internal))
+  (let ((backend (vc-deduce-backend))
+       (default-directory default-directory)
+       rootdir)
+    (if backend
+       (setq rootdir (vc-call-backend backend 'root default-directory))
+      (setq rootdir (read-directory-name "Directory for VC root-log: "))
+      (setq backend (vc-responsible-backend rootdir))
+      (unless backend
+        (error "Directory is not version controlled")))
+    (setq default-directory rootdir)
+    (unless rev2 (setq rev2 "HEAD"))
+    (setq rev1 (vc-call-backend backend 'mergebase rev1 rev2))
+    (vc-print-log-internal backend (list rootdir) rev1 t rev2)))
+
 ;;;###autoload
 (defun vc-region-history (from to)
   "Show the history of the region between FROM and TO.
diff --git a/lisp/vc/vc-git.el b/lisp/vc/vc-git.el
index aa6809f626..82ff2b58e7 100644
--- a/lisp/vc/vc-git.el
+++ b/lisp/vc/vc-git.el
@@ -1045,8 +1045,12 @@ vc-git-print-log
                     ,(format "--pretty=tformat:%s"
                             (car vc-git-root-log-format))
                    "--abbrev-commit"))
-               (when limit (list "-n" (format "%s" limit)))
-               (when start-revision (list start-revision))
+               (when (numberp limit)
+                  (list "-n" (format "%s" limit)))
+               (when start-revision
+                  (if (and limit (not (numberp limit)))
+                      (list (concat start-revision ".." limit))
+                    (list start-revision)))
                '("--")))))))
 
 (defun vc-git-log-outgoing (buffer remote-location)
@@ -1077,6 +1081,12 @@ vc-git-log-incoming
                        "@{upstream}"
                      remote-location))))
 
+(eval-when-compile (require 'subr-x)) ; for string-trim-right
+
+(defun vc-git-mergebase (rev1 &optional rev2)
+  (unless rev2 (setq rev2 "HEAD"))
+  (string-trim-right (vc-git--run-command-string nil "merge-base" rev1 rev2)))
+
 (defvar log-view-message-re)
 (defvar log-view-file-re)
 (defvar log-view-font-lock-keywords)