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

[Juri Linkov]

>> Would it be too disruptive to make "registering" a new file with Git a
>> no-op, and then require the user to commit it by going to VC Dir and
>> marking it?  That's the only alternative I can think of, given that it's
>> common to have lots of untracked, unignored files around you don't
>> intend ever to commit.
>
> I suppose we could use a more complex check in Git's case: if the diff is
> non-empty, check that it doesn't intersect with the diff we want to apply,
> or where it does, the contents are exactly the same. It's more complex than
> the existing one-liner, though.

Maybe like this?

diff --git a/lisp/vc/vc-git.el b/lisp/vc/vc-git.el
index 2941cc75be..792981b142 100644
--- a/lisp/vc/vc-git.el
+++ b/lisp/vc/vc-git.el
@@ -1018,7 +1018,20 @@ vc-git-checkin
                 (make-nearby-temp-file "git-msg")))))
     (when vc-git-patch-string
       (unless (zerop (vc-git-command nil t nil "diff" "--cached" "--quiet"))
-        (user-error "Index not empty"))
+        (with-temp-buffer
+          (vc-git-command (current-buffer) t nil "diff" "--cached")
+          (goto-char (point-min))
+          (let ((pos (point)) file-diff)
+            (forward-line 1)
+            (while (not (eobp))
+              (search-forward "diff --git" nil 'move)
+              (move-beginning-of-line 1)
+              (setq file-diff (buffer-substring pos (point)))
+              (if (string-search file-diff vc-git-patch-string)
+                  (setq vc-git-patch-string
+                        (string-replace file-diff "" vc-git-patch-string))
+                (user-error "Index not empty"))
+              (setq pos (point))))))
       (let ((patch-file (make-temp-file "git-patch")))
         (with-temp-file patch-file
           (insert vc-git-patch-string))