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

builtin-log.c

@@ -932,7 +932,8 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	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)

t/t4014-format-patch.sh

@@ -230,4 +230,29 @@ test_expect_success 'shortlog of cover-letter wraps overly-long onelines' '
 
 '
 
+cat > expect << EOF
+---
+ file |   16 ++++++++++++++++
+ 1 files changed, 16 insertions(+), 0 deletions(-)
+
+diff --git a/file b/file
+index 40f36c6..2dc5c23 100644
+--- a/file
++++ b/file
+@@ -13,4 +13,20 @@ C
+ 10
+ D
+ E
+ F
++5
+EOF
+
+test_expect_success 'format-patch respects -U' '
+
+	git format-patch -U4 -2 &&
+	sed -e "1,/^$/d" -e "/^+5/q" < 0001-This-is-an-excessively-long-subject-line-for-a-messa.patch > output &&
+	test_cmp expect output
+
+'
+
 test_done