Replace most uses of strncmp() with prefixcmp()

This is a preparation for replacing all prefix checks with either
strip_prefix() or starts_with() when Git 1.8.6 is released.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>

ui-shared.c

@@ -120,7 +120,7 @@ const char *cgit_repobasename(const char *reponame)
 	/* strip trailing slashes */
 	while (p && rvbuf[p] == '/') rvbuf[p--] = 0;
 	/* strip trailing .git */
-	if (p >= 3 && !strncmp(&rvbuf[p-3], ".git", 4)) {
+	if (p >= 3 && !prefixcmp(&rvbuf[p-3], ".git")) {
 		p -= 3; rvbuf[p--] = 0;
 	}
 	/* strip more trailing slashes if any */