Add support for automatic and custom clone urls

This adds support for two new parameters to cgitrc: clone-prefix and
repo.clone-url.

If clone-prefix is specified, all repos will get a clone url printed in the
sidebar; the url is generated by clone-prefix + repo.url.

Additionally, each repo can specify repo.clone-url which will override any
such auto-generated url.

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

cgit.h

@@ -75,6 +75,7 @@ struct repoinfo {
 	char *group;
 	char *module_link;
 	char *readme;
+	char *clone_url;
 	int snapshots;
 	int enable_log_filecount;
 	int enable_log_linecount;
@@ -140,6 +141,7 @@ extern char *cgit_script_name;
 extern char *cgit_cache_root;
 extern char *cgit_repo_group;
 extern char *cgit_robots;
+extern char *cgit_clone_prefix;
 
 extern int cgit_nocache;
 extern int cgit_snapshots;