format-patch: make "-p" suppress diffstat

Once upon a time, format-patch would use its default stat
plus patch format only when no diff format was given on the
command line. This meant that "format-patch -p" would
suppress the stat and show just the patch.

Commit 68daa64 changed this to keep the stat format when we
had an "implicit" patch format, like "-U5". As a side
effect, this meant that an explicit patch format was now
ignored (because cmd_format_patch didn't know the reason
that the format was set way down in diff_opt_parse).

This patch unbreaks what 68daa64 did (while still preserving
what 68daa64 was trying to do), reinstating "-p" to suppress
the default behavior. We do this by parsing "-p" ourselves
in format-patch, and noting whether it was used explicitly.

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

t/t4014-format-patch.sh

@@ -455,6 +455,27 @@ test_expect_success 'format-patch respects -U' '
 
 '
 
+cat > expect << EOF
+
+diff --git a/file b/file
+index 40f36c6..2dc5c23 100644
+--- a/file
++++ b/file
+@@ -14,3 +14,19 @@ C
+ D
+ E
+ F
++5
+EOF
+
+test_expect_success 'format-patch -p suppresses stat' '
+
+	git format-patch -p -2 &&
+	sed -e "1,/^$/d" -e "/^+5/q" < 0001-This-is-an-excessively-long-subject-line-for-a-messa.patch > output &&
+	test_cmp expect output
+
+'
+
 test_expect_success 'format-patch from a subdirectory (1)' '
 	filename=$(
 		rm -rf sub &&