git-p4: add options --commit and --disable-rebase

git-p4: add options --commit and --disable-rebase

On a daily work with multiple local git branches, the usual way to
submit only a specified commit was to cherry-pick the commit on
master then run git-p4 submit.  It can be very annoying to switch
between local branches and master, only to submit one commit.  The
proposed new way is to select directly the commit you want to
submit.

Add option --commit to command 'git-p4 submit' in order to submit
only specified commit(s) in p4.

On a daily work developping software with big compilation time, one
may not want to rebase on his local git tree, in order to avoid long
recompilation.

Add option --disable-rebase to command 'git-p4 submit' in order to
disable rebase after submission.

Thanks-to: Cedric Borgese <cedric.borgese@gmail.com>
Reviewed-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Romain Merland <merlorom@yahoo.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>