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-refs.c

@@ -240,9 +240,9 @@ void cgit_print_refs()
 
 	html("<table class='list nowrap'>");
 
-	if (ctx.qry.path && !strncmp(ctx.qry.path, "heads", 5))
+	if (ctx.qry.path && !prefixcmp(ctx.qry.path, "heads"))
 		cgit_print_branches(0);
-	else if (ctx.qry.path && !strncmp(ctx.qry.path, "tags", 4))
+	else if (ctx.qry.path && !prefixcmp(ctx.qry.path, "tags"))
 		cgit_print_tags(0);
 	else {
 		cgit_print_branches(0);