git commit --author=$name: look $name up in existing commits

git commit --author=$name: look $name up in existing commits

This allows "git commit --author=$name" to accept a name that is not in
the required "A U Thor <author@example.xz>" format, and use that to look
up an author name that matches from existing commits.

When using this feature, it is the user's responsibility to give a name
that uniquely matches the name s/he wants, as the logic returns the name
from the first matching commit.

Signed-off-by: Junio C Hamano <gitster@pobox.com>