Add cgit_commit_link() + support for id=sha1 to commit view

This adds a function to generate links to the commit page and extends said
page to use id from querystring as primary revision specified (fallback to
h).

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

cgit.c

@@ -100,7 +100,7 @@ static void cgit_print_repo_page(struct cacheitem *item)
 		cgit_print_tree(cgit_query_sha1, cgit_query_path);
 		break;
 	case CMD_COMMIT:
-		cgit_print_commit(cgit_query_head);
+		cgit_print_commit(cgit_query_sha1);
 		break;
 	case CMD_DIFF:
 		cgit_print_diff(cgit_query_head, cgit_query_sha1, cgit_query_sha2,