bug#67062: 30.0.50; [PATCH] Abbreviate the revision in 'vc-annotate' (for Git)

[Juri Linkov]

> We just want to take that revision and turn it into a friendlier
> (read: shorter) form for the buffer name.

Then the best solution is to introduce a new variable
'vc-short-revision' that vc-annotate should either set as a
buffer-local value or let-bind around the vc backend API call.

Then the git backend could use it optionally depending
on the value of a new user option 'vc-git-short-revision'.

This is much better that adding a new API call.  We don't add
new API calls lightly since any change in API requires updating
the documentation at the top of vc.el and at the top of vc backend
implementation files.