ui-tree.c: add support for path-selected submodule links

The current 'repo.module-link' option is sufficient when all gitlinks
in a repository can be converted to commit links in a uniform way, but
not when different submodules/paths needs different settings.

This patch adds support for 'repo.module-link.<path>', which will be
used for linking to submodules at paths matching one such entry.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgit.h

@@ -88,6 +88,7 @@ struct cgit_repo {
 	struct cgit_filter *about_filter;
 	struct cgit_filter *commit_filter;
 	struct cgit_filter *source_filter;
+	struct string_list submodules;
 };
 
 typedef void (*repo_config_fn)(struct cgit_repo *repo, const char *name,