gitweb: link to "git describe"'d commits in log messages

gitweb: link to "git describe"'d commits in log messages

Change the log formatting function to know about "git describe" output
such as "v2.8.0-4-g867ad08", in addition to just plain "867ad08".

There are still many valid refnames that we don't link to
e.g. v2.10.0-rc1~2^2~1 is also a valid way to refer to
v2.8.0-4-g867ad08, but I'm not supporting that with this commit,
similarly it's trivially possible to create some refnames like
"æ/var-gf6727b0" or which won't be picked up by this regex.

There's surely room for improvement here, but I just wanted to address
the very common case of sticking "git describe" output into commit
messages without trying to link to all possible refnames, that's going
to be a rather futile exercise given that this is free text, and it
would be prohibitively expensive to look up whether the references in
question exist in our repository.

There was on-list discussion about how we could do better than this
patch. Junio suggested to update parse_commits() to call a new
"gitweb--helper" command which would pass each of the revision
candidates through "rev-parse --verify --quiet". That would cut down
on our false positives (e.g. we'll link to "deadbeef"), and also allow
us to be more aggressive in selecting candidate revisions.

That may be too expensive to work in practice, or it may
not. Investigating that would be a good follow-up to this patch.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Acked-by: Jakub Narębski <redacted>
Signed-off-by: Junio C Hamano <redacted>