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.
>> +      (unless (eq (vc-git-command nil t nil "diff" "--cached" "--quiet") 0)
>> +        (error "Index not empty"))
>
> user-error, maybe?

Oh, it has a sad shortcoming: this feature can't be used when a new file
is added with 'C-x v v' since new files get added directly to the index,
and checking for the empty index raises the error for the whole changeset
with modified old files and new files.