pull: handle --log=<n>

pull: handle --log=<n>

Since efb779f (merge, pull: add '--(no-)log' command line option,
2008-04-06) git-pull supported the (--no-)log switch and would pass it
to git-merge.

96e9420 (merge: Make '--log' an integer option for number of shortlog
entries, 2010-09-08) implemented support for the --log=<n> switch, which
would explicitly set the number of shortlog entries. However, git-pull
does not recognize this option, and will instead pass it to git-fetch,
leading to "unknown option" errors.

Fix this by matching --log=* in addition to --log and --no-log.

Implement a test for this use case.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>