format-patch: output header for empty commits

When formatting an empty commit, it is surprising that a totally empty
file is generated.  Set the flag to always print the header, matching
the behaviour of git-log.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/log.c

@@ -2097,6 +2097,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 
 	/* Always generate a patch */
 	rev.diffopt.output_format |= DIFF_FORMAT_PATCH;
+	rev.always_show_header = 1;
 
 	rev.zero_commit = zero_commit;
 	rev.patch_name_max = fmt_patch_name_max;

t/t4014-format-patch.sh

@@ -59,6 +59,10 @@ test_expect_success setup '
 	test_tick &&
 	git commit -m "patchid 3" &&
 
+	git checkout -b empty main &&
+	test_tick &&
+	git commit --allow-empty -m "empty commit" &&
+
 	git checkout main
 '
 
@@ -128,6 +132,12 @@ test_expect_success 'replay did not screw up the log message' '
 	grep "^Side .* with .* backslash-n" actual
 '
 
+test_expect_success 'format-patch empty commit' '
+	git format-patch --stdout main..empty >empty &&
+	grep "^From " empty >from &&
+	test_line_count = 1 from
+'
+
 test_expect_success 'extra headers' '
 	git config format.headers "To: R E Cipient <rcipient@example.com>
 " &&