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>

shared.c

@@ -63,6 +63,7 @@ struct cgit_repo *cgit_add_repo(const char *url)
 	ret->enable_remote_branches = ctx.cfg.enable_remote_branches;
 	ret->enable_subject_links = ctx.cfg.enable_subject_links;
 	ret->max_stats = ctx.cfg.max_stats;
+	ret->branch_sort = ctx.cfg.branch_sort;
 	ret->commit_sort = ctx.cfg.commit_sort;
 	ret->module_link = ctx.cfg.module_link;
 	ret->readme = ctx.cfg.readme;