Add prefix parameter to cgit_diff_tree()

This paramter can be used to restrict a diff to the specified path prefix.

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

cgit.h

@@ -170,7 +170,7 @@ extern int cgit_diff_files(const unsigned char *old_sha1,
 
 extern void cgit_diff_tree(const unsigned char *old_sha1,
 			   const unsigned char *new_sha1,
-			   filepair_fn fn);
+			   filepair_fn fn, const char *prefix);
 
 extern void cgit_diff_commit(struct commit *commit, filepair_fn fn);