format-patch: add ability to use alt cover format

Often when sending patch series there's a need to clarify to the
reviewer what's the purpose of said series, since it might be difficult
to understand it from reading the commits messages one by one.

"git format-patch" provides the useful "--cover-letter" flag to declare
if we want it to generate a template for us to use. By default it will
generate a "git shortlog" of the changes, which developers find less
useful than they'd like, mainly because the shortlog groups commits by
author, and gives no obvious chronological order.

Give the ability to format-patch to specify an alternative format spec
through the "--cover-letter-format" option. This option either takes
"shortlog", which is the current format, or a format spec prefixed with
"log:".

Example:
    git format-patch --cover-letter \
        --cover-letter-format="log:[%(count)/%(total)] %s (%an)" HEAD~3

    [1/3] this is a commit summary (Mirko Faina)
    [2/3] this is another commit summary (Mirko Faina)
    ...

Signed-off-by: Mirko Faina <mroik@delayed.space>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/log.c

@@ -1343,13 +1343,36 @@ static void generate_shortlog_cover_letter(struct shortlog *log,
 	shortlog_output(log);
 }
 
+static void generate_commit_list_cover(FILE *cover_file,const char *format,
+				       struct commit **list, int n)
+{
+	struct strbuf commit_line = STRBUF_INIT;
+	struct pretty_print_context ctx = {0};
+	struct rev_info rev = REV_INFO_INIT;
+
+	strbuf_init(&commit_line, 0);
+	rev.total = n;
+	ctx.rev = &rev;
+	for (int i = n - 1; i >= 0; i--) {
+		rev.nr = n - i;
+		repo_format_commit_message(the_repository, list[i], format,
+				&commit_line, &ctx);
+		fprintf(cover_file, "%s\n", commit_line.buf);
+		strbuf_reset(&commit_line);
+	}
+	fprintf(cover_file, "\n");
+
+	strbuf_release(&commit_line);
+}
+
 static void make_cover_letter(struct rev_info *rev, int use_separate_file,
 			      struct commit *origin,
 			      int nr, struct commit **list,
 			      const char *description_file,
 			      const char *branch_name,
 			      int quiet,
-			      const struct format_config *cfg)
+			      const struct format_config *cfg,
+			      const char *format)
 {
 	const char *committer;
 	struct shortlog log;
@@ -1396,7 +1419,12 @@ static void make_cover_letter(struct rev_info *rev, int use_separate_file,
 	free(pp.after_subject);
 	strbuf_release(&sb);
 
-	generate_shortlog_cover_letter(&log, rev, list, nr);
+	if (skip_prefix(format, "log:", &format))
+		generate_commit_list_cover(rev->diffopt.file, format, list, nr);
+	else if (!strcmp(format, "shortlog"))
+		generate_shortlog_cover_letter(&log, rev, list, nr);
+	else
+		die(_("'%s' is not a valid format string"), format);
 
 	/* We can only do diffstat with a unique reference point */
 	if (origin)
@@ -1914,6 +1942,7 @@ int cmd_format_patch(int argc,
 	int just_numbers = 0;
 	int ignore_if_in_upstream = 0;
 	int cover_letter = -1;
+	const char *cover_letter_fmt = NULL;
 	int boundary_count = 0;
 	int no_binary_diff = 0;
 	int zero_commit = 0;
@@ -1960,6 +1989,8 @@ int cmd_format_patch(int argc,
 			    N_("print patches to standard out")),
 		OPT_BOOL(0, "cover-letter", &cover_letter,
 			    N_("generate a cover letter")),
+		OPT_STRING(0, "cover-letter-format", &cover_letter_fmt, N_("format-spec"),
+			    N_("format spec used for the commit list in the cover letter")),
 		OPT_BOOL(0, "numbered-files", &just_numbers,
 			    N_("use simple number sequence for output file names")),
 		OPT_STRING(0, "suffix", &fmt_patch_suffix, N_("sfx"),
@@ -2297,6 +2328,7 @@ int cmd_format_patch(int argc,
 		/* nothing to do */
 		goto done;
 	total = list.nr;
+
 	if (cover_letter == -1) {
 		if (cfg.config_cover_letter == COVER_AUTO)
 			cover_letter = (total > 1);
@@ -2383,12 +2415,14 @@ int cmd_format_patch(int argc,
 	}
 	rev.numbered_files = just_numbers;
 	rev.patch_suffix = fmt_patch_suffix;
+
 	if (cover_letter) {
 		if (cfg.thread)
 			gen_message_id(&rev, "cover");
 		make_cover_letter(&rev, !!output_directory,
 				  origin, list.nr, list.items,
-				  description_file, branch_name, quiet, &cfg);
+				  description_file, branch_name, quiet, &cfg,
+				  cover_letter_fmt);
 		print_bases(&bases, rev.diffopt.file);
 		print_signature(signature, rev.diffopt.file);
 		total++;

t/t4014-format-patch.sh

@@ -380,6 +380,54 @@ test_expect_success 'filename limit applies only to basename' '
 	done
 '
 
+test_expect_success 'cover letter with subject, author and count' '
+	rm -rf patches &&
+	test_when_finished "git reset --hard HEAD~1" &&
+	test_when_finished "rm -rf patches result test_file" &&
+	touch test_file &&
+	git add test_file &&
+	git commit -m "This is a subject" &&
+	git format-patch --cover-letter \
+	--cover-letter-format="log:[%(count)/%(total)] %s (%an)" -o patches HEAD~1 &&
+	grep "^\[1/1\] This is a subject (A U Thor)$" patches/0000-cover-letter.patch >result &&
+	test_line_count = 1 result
+'
+
+test_expected_success 'cover letter with author and count' '
+	test_when_finished "git reset --hard HEAD~1" &&
+	test_when_finished "rm -rf patches result test_file" &&
+	touch test_file &&
+	git add test_file &&
+	git commit -m "This is a subject" &&
+	git format-patch --cover-letter \
+	--cover-letter-format="log:[%(count)/%(total)] %an" -o patches HEAD~1 &&
+	grep "^\[1/1\] A U Thor$" patches/0000-cover-letter.patch >result &&
+	test_line_count = 1 result
+'
+
+test_expect_success 'cover letter shortlog' '
+	test_when_finished "git reset --hard HEAD~1" &&
+	test_when_finished "rm -rf patches result test_file" &&
+	touch test_file &&
+	git add test_file &&
+	git commit -m "This is a subject" &&
+	git format-patch --cover-letter --cover-letter-format=shortlog \
+	-o patches HEAD~1 &&
+	sed -n -e "/^A U Thor/p;" patches/0000-cover-letter.patch >result &&
+	test_line_count = 1 result
+'
+
+test_expect_success 'cover letter no format' '
+	test_when_finished "git reset --hard HEAD~1" &&
+	test_when_finished "rm -rf patches result test_file" &&
+	touch test_file &&
+	git add test_file &&
+	git commit -m "This is a subject" &&
+	git format-patch --cover-letter -o patches HEAD~1 &&
+	sed -n -e "/^A U Thor/p;" patches/0000-cover-letter.patch >result &&
+	test_line_count = 1 result
+'
+
 test_expect_success 'reroll count' '
 	rm -fr patches &&
 	git format-patch -o patches --cover-letter --reroll-count 4 main..side >list &&

t/t9902-completion.sh

@@ -2774,6 +2774,7 @@ test_expect_success PERL 'send-email' '
 	test_completion "git send-email --cov" <<-\EOF &&
 	--cover-from-description=Z
 	--cover-letter Z
+	--cover-letter-format=Z
 	EOF
 	test_completion "git send-email --val" <<-\EOF &&
 	--validate Z