bug#52349: 29.0.50; vc-git and diff-mode: stage hunks

[Juri Linkov]

>>> Perhaps we should also check that there are no existing staged changes for
>>> those files, and if so, abort? This way we won't commit anything else by
>>> accident.
>> Will do later.
>
> Shouldn't be too hard, if we don't try to do it eagerly. Just add a check
> inside (when vc-git-patch-string ...) in vc-git-checkin.

This uses the same command `git diff --cached --quiet` as in commit-patch
that you mentioned earlier
https://github.com/caldwell/commit-patch/blob/master/commit-patch#L215-L217
(but with an error message shorter than "Cowardly refusing to do anything
potentially harmful":)

diff --git a/lisp/vc/vc-git.el b/lisp/vc/vc-git.el
index 8458b40714..fa09ecf472 100644
--- a/lisp/vc/vc-git.el
+++ b/lisp/vc/vc-git.el
@@ -944,6 +944,8 @@ vc-git-checkin
               (let ((default-directory (file-name-directory file1)))
                 (make-nearby-temp-file "git-msg")))))
     (when vc-git-patch-string
+      (unless (eq (vc-git-command nil t nil "diff" "--cached" "--quiet") 0)
+        (error "Index not empty"))
       (let ((patch-file (make-temp-file "git-patch")))
         (with-temp-file patch-file
           (insert vc-git-patch-string))