Document pre-condition for tree_entry_interesting

tree_entry_interesting will fail to find appropriate matches if the base
directory path is not terminated with a slash.  Knowing this earlier would
have saved me some debugging time.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

tree-diff.c

@@ -85,6 +85,8 @@ static int compare_tree_entry(struct tree_desc *t1, struct tree_desc *t2, const
 /*
  * Is a tree entry interesting given the pathspec we have?
  *
+ * Pre-condition: baselen == 0 || base[baselen-1] == '/'
+ *
  * Return:
  *  - 2 for "yes, and all subsequent entries will be"
  *  - 1 for yes