branch: implement '--format' option

Implement the '--format' option provided by 'ref-filter'. This lets the
user list branches as per desired format similar to the implementation
in 'git for-each-ref'.

Add tests and documentation for the same.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/branch.c b/builtin/branch.c
index f293ee5..cbaa6d0 100644
--- a/builtin/branch.c
+++ b/builtin/branch.c
@@ -28,6 +28,7 @@ static const char * const builtin_branch_usage[] = {
 	N_("git branch [<options>] [-r] (-d | -D) <branch-name>..."),
 	N_("git branch [<options>] (-m | -M) [<old-branch>] <new-branch>"),
 	N_("git branch [<options>] [-r | -a] [--points-at]"),
+	N_("git branch [<options>] [-r | -a] [--format]"),
 	NULL
 };
 
@@ -364,14 +365,14 @@ static char *build_format(struct ref_filter *filter, int maxwidth, const char *r
 	return strbuf_detach(&fmt, NULL);
 }
 
-static void print_ref_list(struct ref_filter *filter, struct ref_sorting *sorting)
+static void print_ref_list(struct ref_filter *filter, struct ref_sorting *sorting, const char *format)
 {
 	int i;
 	struct ref_array array;
 	int maxwidth = 0;
 	const char *remote_prefix = "";
 	struct strbuf out = STRBUF_INIT;
-	char *format;
+	char *to_free = NULL;
 
 	/*
 	 * If we are listing more than just remote branches,
@@ -388,7 +389,8 @@ static void print_ref_list(struct ref_filter *filter, struct ref_sorting *sortin
 	if (filter->verbose)
 		maxwidth = calc_maxwidth(&array, strlen(remote_prefix));
 
-	format = build_format(filter, maxwidth, remote_prefix);
+	if (!format)
+		format = to_free = build_format(filter, maxwidth, remote_prefix);
 	verify_ref_format(format);
 
 	ref_array_sort(sorting, &array);
@@ -407,7 +409,7 @@ static void print_ref_list(struct ref_filter *filter, struct ref_sorting *sortin
 	}
 
 	ref_array_clear(&array);
-	free(format);
+	free(to_free);
 }
 
 static void reject_rebase_or_bisect_branch(const char *target)
@@ -528,6 +530,7 @@ int cmd_branch(int argc, const char **argv, const char *prefix)
 	struct ref_filter filter;
 	int icase = 0;
 	static struct ref_sorting *sorting = NULL, **sorting_tail = &sorting;
+	const char *format = NULL;
 
 	struct option options[] = {
 		OPT_GROUP(N_("Generic options")),
@@ -569,6 +572,7 @@ int cmd_branch(int argc, const char **argv, const char *prefix)
 			N_("print only branches of the object"), 0, parse_opt_object_name
 		},
 		OPT_BOOL('i', "ignore-case", &icase, N_("sorting and filtering are case insensitive")),
+		OPT_STRING(  0 , "format", &format, N_("format"), N_("format to use for the output")),
 		OPT_END(),
 	};
 
@@ -641,7 +645,7 @@ int cmd_branch(int argc, const char **argv, const char *prefix)
 		if (!sorting)
 			sorting = ref_default_sorting();
 		sorting->ignore_case = icase;
-		print_ref_list(&filter, sorting);
+		print_ref_list(&filter, sorting, format);
 		print_columns(&output, colopts, NULL);
 		string_list_clear(&output, 0);
 		return 0;