git-p4: add option to preserve user names

git-p4: add option to preserve user names

Patches from git passed into p4 end up with the committer being identified
as the person who ran git-p4.

With "submit --preserve-user", git-p4 modifies the p4 changelist (after it
has been submitted), setting the p4 author field.

The submitter is required to have sufficient p4 permissions or git-p4
refuses to proceed. If the git author is not known to p4, the submit will
be abandoned unless git-p4.allowMissingP4Users is true.

Signed-off-by: Luke Diamand <luke@diamand.org>
Acked-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>