git-p4: Improve rename detection support

git-p4: Improve rename detection support

Only open files for edit after integrating if the SHA1 of source and destination
differ from each other.
Add git config option detectRenames to allow permanent rename detection. This
options should be set to a true/false value.
Rename "detectRename" variable to "detectRenames" to make it more coherent with
the description in git man pages, which always use plural.

Signed-off-by: Vitor Antunes <vitor.hda@gmail.com>
Acked-by: Pete Wyckoff <pw@padd.com>
Acked-by: Tor Arvid Lund <torarvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>