pathspec: rename free_pathspec() to clear_pathspec()

The function takes a pointer to a pathspec structure, and releases
the resources held by it, but does not free() the structure itself.
Such a function should be called "clear", not "free".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

combine-diff.c

@@ -1525,7 +1525,7 @@ void diff_tree_combined(const unsigned char *sha1,
 		free(tmp);
 	}
 
-	free_pathspec(&diffopts.pathspec);
+	clear_pathspec(&diffopts.pathspec);
 }
 
 void diff_tree_combined_merge(const struct commit *commit, int dense,