log: add %S option (like --source) to log --format

Make it possible to write for example

        git log --format="%H,%S"

where the %S at the end is a new placeholder that prints out the ref
(tag/branch) for each commit.

Using %d might seem like an alternative but it only shows the ref for the last
commit in the branch.

Signed-off-by: Issac Trotts <issactrotts@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -134,6 +134,8 @@ The placeholders are:
 - '%cI': committer date, strict ISO 8601 format
 - '%d': ref names, like the --decorate option of linkgit:git-log[1]
 - '%D': ref names without the " (", ")" wrapping.
+- '%S': ref name given on the command line by which the commit was reached
+  (like `git log --source`), only works with `git log`
 - '%e': encoding
 - '%s': subject
 - '%f': sanitized subject line, suitable for a filename

builtin/log.c

@@ -203,7 +203,7 @@ static void cmd_log_init_finish(int argc, const char **argv, const char *prefix,
 	    rev->diffopt.filter || rev->diffopt.flags.follow_renames)
 		rev->always_show_header = 0;
 
-	if (source) {
+	if (source || w.source) {
 		init_revision_sources(&revision_sources);
 		rev->sources = &revision_sources;
 	}

log-tree.c

@@ -700,6 +700,7 @@ void show_log(struct rev_info *opt)
 	ctx.color = opt->diffopt.use_color;
 	ctx.expand_tabs_in_log = opt->expand_tabs_in_log;
 	ctx.output_encoding = get_log_output_encoding();
+	ctx.rev = opt;
 	if (opt->from_ident.mail_begin && opt->from_ident.name_begin)
 		ctx.from_ident = &opt->from_ident;
 	if (opt->graph)

pretty.c

@@ -1084,6 +1084,7 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 	struct commit_list *p;
 	const char *arg;
 	int ch;
+	char **slot;
 
 	/* these are independent of the commit */
 	switch (placeholder[0]) {
@@ -1194,6 +1195,14 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 		load_ref_decorations(NULL, DECORATE_SHORT_REFS);
 		format_decorations_extended(sb, commit, c->auto_color, "", ", ", "");
 		return 1;
+	case 'S':		/* tag/branch like --source */
+		if (!(c->pretty_ctx->rev && c->pretty_ctx->rev->sources))
+			return 0;
+		slot = revision_sources_at(c->pretty_ctx->rev->sources, commit);
+		if (!(slot && *slot))
+			return 0;
+		strbuf_addstr(sb, *slot);
+		return 1;
 	case 'g':		/* reflog info */
 		switch(placeholder[1]) {
 		case 'd':	/* reflog selector */
@@ -1498,6 +1507,9 @@ static size_t userformat_want_item(struct strbuf *sb, const char *placeholder,
 	case 'N':
 		w->notes = 1;
 		break;
+	case 'S':
+		w->source = 1;
+		break;
 	}
 	return 0;
 }

pretty.h

@@ -60,6 +60,7 @@ static inline int cmit_fmt_is_mail(enum cmit_fmt fmt)
 
 struct userformat_want {
 	unsigned notes:1;
+	unsigned source:1;
 };
 
 /* Set the flag "w->notes" if there is placeholder %N in "fmt". */

t/t4205-log-pretty-formats.sh

@@ -621,4 +621,54 @@ test_expect_success 'trailer parsing not fooled by --- line' '
 	test_cmp expect actual
 '
 
+test_expect_success 'set up %S tests' '
+	git checkout --orphan source-a &&
+	test_commit one &&
+	test_commit two &&
+	git checkout -b source-b HEAD^ &&
+	test_commit three
+'
+
+test_expect_success 'log --format=%S paints branch names' '
+	cat >expect <<-\EOF &&
+	source-b
+	source-a
+	source-b
+	EOF
+	git log --format=%S source-a source-b >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'log --format=%S paints tag names' '
+	git tag -m tagged source-tag &&
+	cat >expect <<-\EOF &&
+	source-tag
+	source-a
+	source-tag
+	EOF
+	git log --format=%S source-tag source-a >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'log --format=%S paints symmetric ranges' '
+	cat >expect <<-\EOF &&
+	source-b
+	source-a
+	EOF
+	git log --format=%S source-a...source-b >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success '%S in git log --format works with other placeholders (part 1)' '
+	git log --format="source-b %h" source-b >expect &&
+	git log --format="%S %h" source-b >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success '%S in git log --format works with other placeholders (part 2)' '
+	git log --format="%h source-b" source-b >expect &&
+	git log --format="%h %S" source-b >actual &&
+	test_cmp expect actual
+'
+
 test_done

t/t6006-rev-list-format.sh

@@ -185,6 +185,10 @@ test_expect_success 'basic colors' '
 	test_cmp expect actual
 '
 
+test_expect_success '%S is not a placeholder for rev-list yet' '
+	git rev-list --format="%S" -1 master | grep "%S"
+'
+
 test_expect_success 'advanced colors' '
 	cat >expect <<-EOF &&
 	commit $head2