git-svn: add support for --first-parent

git-svn: add support for --first-parent

When git-svn uses git-log to find embedded 'git-svn-id'-lines in commit
messages, it can get confused when local history contains merges with
other git-svn branches. But if --first-parent is supplied to git-log,
working_head_info() will only see 'branch-local' commits and thus the
first commit containing a 'git-svn-id' line should refer to the correct
subversion branch.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>