git-p4: warn if git authorship won't be retained

git-p4: warn if git authorship won't be retained

If the git commits you are submitting contain changes made by
other people, the authorship will not be retained. Change git-p4
to warn of this and to note that --preserve-user can be used
to solve the problem (if you have suitable permissions).
The warning can be disabled.

Add a test case and update documentation.

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