bug#2982: 23.0.92; `vc-diff' behaves inconsistently with git

[Markus Triska <markus.triska@gmx.at>]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

> So when you open the file, Emacs asks `git' the status of the file and
> so knows that this modtime corresponds to "up-to-date".  After `touch',
> OTOH, Emacs isn't so sure any more, so it runs `git diff'

OK, external modtime changes may be harder to handle than an Emacs use
case: Say you resume working on a file with uncommitted changes, and you
first do vc-diff and decide to throw them away. Then in the *vc-diff*
buffer, you do C-u C-c C-a until all chunks are reverted, then save the
file and see if really all changes are gone with C-x v =. At this point
the *vc-diff* buffer is shown unexpectedly without remaining changes.