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>
diff --git a/builtin-log.c b/builtin-log.c
index 9204ffd..1d3c5cb 100644
--- a/builtin-log.c
+++ b/builtin-log.c
@@ -932,7 +932,8 @@
 	if (argc > 1)
 		die ("unrecognized argument: %s", argv[1]);
 
-	if (!rev.diffopt.output_format)
+	if (!rev.diffopt.output_format
+		|| rev.diffopt.output_format == DIFF_FORMAT_PATCH)
 		rev.diffopt.output_format = DIFF_FORMAT_DIFFSTAT | DIFF_FORMAT_SUMMARY | DIFF_FORMAT_PATCH;
 
 	if (!DIFF_OPT_TST(&rev.diffopt, TEXT) && !no_binary_diff)