bug#59011: 29.0.50; change to vc-git-working-revision breaks vc-git-mode-line-string

[Philip Kaludercic]

Jonas Bernoulli <jonas@bernoul.li> writes:

> Since 307ad210040251ea0de2e7f453350c4497bda874 `vc-git-working-revision'
> may return the name of a reference (aka symbolic commit) instead of the
> full sha1 hash, as it used to always do.
>
> When that name is shorter than seven characters, then that change breaks
> `vc-git-mode-line-string', which expects to be able to extract the first
> seven characters of the string returned by `vc-git-working-revision'.

This was already touched on in bug#58709, and I believe it is best to
revert the change for now.

The motivation for the change was to make selecting revisions for the
new `vc-prepare-patch' command, but perhaps it would be better to handle
the issue somewhere else like in vc-*-revision-table.

> (By the way, it is not correct to abbreviate a commit hash to the first
> N characters using (substring hash 0 N).  Even in a small repository
> there is a risk that such a naive abbreviation is ambiguous.  Instead of
> doing that, one should use Git to perform the abbreviation; that way one
> can request an abbreviation of a certain length, and Git will comply --
> unless doing that would be ambiguous, in which case it returns a longer
> abbreviation.)

If this is only done in the mode line, then I don't think there is too
much harm in doing so (it isn't fed back into git at any point), though
using a proper abbreviation would technically preferable.