format-patch: add an option to suppress commit hash

Oftentimes, patches created by git format-patch will be stored in
version control or compared with diff.  In these cases, two otherwise
identical patches can have different commit hashes, leading to diff
noise.  Teach git format-patch a --zero-commit option that instead
produces an all-zero hash to avoid this diff noise.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-format-patch.txt

@@ -256,6 +256,10 @@ you can use `--suffix=-patch` to get `0001-description-of-my-change-patch`.
 	using this option cannot be applied properly, but they are
 	still useful for code review.
 
+--zero-commit::
+  Output an all-zero hash in each patch's From header instead
+  of the hash of the commit.
+
 --root::
 	Treat the revision argument as a <revision range>, even if it
 	is just a single commit (that would normally be treated as a

builtin/log.c

@@ -1196,6 +1196,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	int cover_letter = -1;
 	int boundary_count = 0;
 	int no_binary_diff = 0;
+	int zero_commit = 0;
 	struct commit *origin = NULL;
 	const char *in_reply_to = NULL;
 	struct patch_ids ids;
@@ -1236,6 +1237,8 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 			    PARSE_OPT_NOARG | PARSE_OPT_NONEG, keep_callback },
 		OPT_BOOL(0, "no-binary", &no_binary_diff,
 			 N_("don't output binary diffs")),
+		OPT_BOOL(0, "zero-commit", &zero_commit,
+			 N_("output all-zero hash in From header")),
 		OPT_BOOL(0, "ignore-if-in-upstream", &ignore_if_in_upstream,
 			 N_("don't include a patch matching a commit upstream")),
 		{ OPTION_SET_INT, 'p', "no-stat", &use_patch_format, NULL,
@@ -1380,6 +1383,8 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	/* Always generate a patch */
 	rev.diffopt.output_format |= DIFF_FORMAT_PATCH;
 
+	rev.zero_commit = zero_commit;
+
 	if (!DIFF_OPT_TST(&rev.diffopt, TEXT) && !no_binary_diff)
 		DIFF_OPT_SET(&rev.diffopt, BINARY);
 

log-tree.c

@@ -342,7 +342,8 @@ void log_write_email_headers(struct rev_info *opt, struct commit *commit,
 {
 	const char *subject = NULL;
 	const char *extra_headers = opt->extra_headers;
-	const char *name = oid_to_hex(&commit->object.oid);
+	const char *name = oid_to_hex(opt->zero_commit ?
+				      &null_oid : &commit->object.oid);
 
 	*need_8bit_cte_p = 0; /* unknown */
 	if (opt->total > 0) {

revision.h

@@ -135,6 +135,7 @@ struct rev_info {
 			pretty_given:1,
 			abbrev_commit:1,
 			abbrev_commit_given:1,
+			zero_commit:1,
 			use_terminator:1,
 			missing_newline:1,
 			date_mode_explicit:1,

t/t4014-format-patch.sh

@@ -1431,4 +1431,11 @@ test_expect_success 'cover letter auto user override' '
 	test_line_count = 2 list
 '
 
+test_expect_success 'format-patch --zero-commit' '
+	git format-patch --zero-commit --stdout v2..v1 >patch2 &&
+	grep "^From " patch2 | sort | uniq >actual &&
+	echo "From $_z40 Mon Sep 17 00:00:00 2001" >expect &&
+	test_cmp expect actual
+'
+
 test_done