format-patch: use default diff format even with patch options

format-patch: use default diff format even with patch options

Previously, running "git format-patch -U5" would cause the
low-level diff machinery to change the diff output format
from "not specified" to "patch". This meant that
format-patch thought we explicitly specified a diff output
format, and would not use the default format. The resulting
message lacked both the diffstat and the summary, as well as
the separating "---".

Now format-patch explicitly checks for this condition and
uses the default. That means that "git format-patch -p" will
now have the "-p" ignored.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>