Document new --date=<format>
Document new --date=<format>

Now, git-log family can take full range of internally supported date format
to their --date=<format> argument.  Document it.

Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git