Add basic diff view

Finally, xdiff is used to show per-file diffs via commit view.

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

cgit.c

@@ -36,6 +36,8 @@ static void cgit_print_repo_page(struct cacheitem *item)
 		cgit_print_commit(cgit_query_sha1);
 	} else if (!strcmp(cgit_query_page, "view")) {
 		cgit_print_view(cgit_query_sha1);
+	} else if (!strcmp(cgit_query_page, "diff")) {
+		cgit_print_diff(cgit_query_sha1, cgit_query_sha2);
 	}
 	cgit_print_docend();
 }