ui-repolist: implement lazy caching of repo->mtime

When sorting the list of repositories by their last modification time,
cgit would (in the worst case) invoke fstat(3) four times and open(3)
twice for each callback from qsort(3). This obviously scales very badly.

Now, the calculated modtime for each repo is saved in repo->mtime, thus
keeping the number of stat/open invocations identical for sorted and
unsorted repo-listings.

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

ui-repolist.c

@@ -32,19 +32,27 @@ static int get_repo_modtime(const struct cgit_repo *repo, time_t *mtime)
 {
 	char *path;
 	struct stat s;
+	struct cgit_repo *r = (struct cgit_repo *)repo;
 
+	if (repo->mtime != -1) {
+		*mtime = repo->mtime;
+		return 1;
+	}
 	path = fmt("%s/%s", repo->path, ctx.cfg.agefile);
 	if (stat(path, &s) == 0) {
 		*mtime = read_agefile(path);
+		r->mtime = *mtime;
 		return 1;
 	}
 
 	path = fmt("%s/refs/heads/%s", repo->path, repo->defbranch);
-	if (stat(path, &s) == 0) {
+	if (stat(path, &s) == 0)
 		*mtime = s.st_mtime;
-		return 1;
-	}
-	return 0;
+	else
+		*mtime = 0;
+
+	r->mtime = *mtime;
+	return (r->mtime != 0);
 }
 
 static void print_modtime(struct cgit_repo *repo)