gitweb: link to 7-char+ SHA-1s, not only 8-char+

gitweb: link to 7-char+ SHA-1s, not only 8-char+

Change the minimum length of an abbreviated object identifier in the
commit message gitweb tries to turn into link from 8 hexchars to 7.

This arbitrary minimum length of 8 was introduced in bfe2191 ("gitweb:
SHA-1 in commit log message links to "object" view", 2006-12-10), but
the default abbreviation length is 7, and has been for a long time.

It's still possible to reference SHA-1s down to 4 characters in length,
see v1.7.4-1-gdce9648's MINIMUM_ABBREV, but I can't see how to make
git actually produce that, so I doubt anyone is putting that into log
messages in practice, but people definitely do put 7 character SHA-1s
into log messages.

I think it's fairly dubious to link to things matching [0-9a-fA-F]
here as opposed to just [0-9a-f], that dates back to the initial
version of gitweb from 161332a ("first working version",
2005-08-07). Git will accept all-caps SHA-1s, but didn't ever produce
them as far as I can tell.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Acked-by: Jakub Narębski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>