Fix "--abbrev=xyz" for revision listing

Fix "--abbrev=xyz" for revision listing

The revision argument parsing was happily parsing "--abbrev", but it
didn't parse "--abbrev=<n>".

Which was hidden by the fact that the diff options _would_ parse
--abbrev=<n>, so it would actually silently parse it, it just
wouldn't use it for the same things that a plain "--abbrev" was
used for.

Which seems a bit insane.

With this patch, if you do "git log --abbrev=10" it will abbreviate the
merge parent commit ID's to ten hex characters, which was probably what
you expected.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>