ui-shared: use strcmp() to compare strings

Somehow a couple of pointer comparisions crept into the new repolink()
and cgit_tree_link() functions. Fix it.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
diff --git a/ui-shared.c b/ui-shared.c
index 9ab6409..657e8af 100644
--- a/ui-shared.c
+++ b/ui-shared.c
@@ -127,7 +127,7 @@
 			html_attr(path);
 		delim = "&amp;";
 	}
-	if (head && head != cgit_query_head) {
+	if (head && strcmp(head, cgit_query_head)) {
 		html(delim);
 		html("h=");
 		html_attr(head);
@@ -142,7 +142,7 @@
 	char *delim;
 
 	delim = repolink(title, class, "tree", head, path);
-	if (rev && rev != cgit_query_head) {
+	if (rev && strcmp(rev, cgit_query_head)) {
 		html(delim);
 		html("id=");
 		html_attr(rev);