Add branch-sort and repo.branch-sort options.

When set to "name", branches are sorted by name, which is the current
default. When set to "age", branches are sorted by the age of the
repository.

This feature was requested by Konstantin Ryabitsev for use on
kernel.org.

Proposed-by: Konstantin Ryabitsev <mricon@kernel.org>

cgit.h

@@ -85,6 +85,7 @@ struct cgit_repo {
 	int enable_remote_branches;
 	int enable_subject_links;
 	int max_stats;
+	int branch_sort;
 	int commit_sort;
 	time_t mtime;
 	struct cgit_filter *about_filter;
@@ -234,6 +235,7 @@ struct cgit_config {
 	int summary_log;
 	int summary_tags;
 	int ssdiff;
+	int branch_sort;
 	int commit_sort;
 	struct string_list mimetypes;
 	struct cgit_filter *about_filter;