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

[Stefan Monnier]

>> In the case of git, it would be easier to ask git the status
> Exactly, after what you explained before, it seems to me that Emacs
> should ask git for the status of the file right after saving as well,
> and store the new modification time (you explained that it already does
> it after opening). Of course, this does not solve the problem of
> external modtime changes, but it seems that this is solvable as well:
> Before running diff, Emacs could ask git whether the status has changed.
> It seems that this would yield consistent behaviour of vc-diff
> regardless of whether the file was just opened or saved in between.

Yes, doing it just before diff might be OK.  Patches welcome.  Note that
it may turn out to be trickier than it seems because of the division
between the generic VC code and the Git-specific one: any change to the
generic code may have nasty consequences on other backends where
`status' may take a non-negligible amount of time.


        Stefan