dir: add commentary explaining match_pathspec_item's return value

The way match_pathspec_item() handles names and pathspecs with trailing
slash characters, in conjunction with special options like
DO_MATCH_DIRECTORY and DO_MATCH_LEADING_PATHSPEC were non-obvious, and
broken until this patch series.  Add a table in a comment explaining the
intent of how these work.

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

dir.c

@@ -276,16 +276,27 @@ static int do_read_blob(const struct object_id *oid, struct oid_stat *oid_stat,
 #define DO_MATCH_LEADING_PATHSPEC (1<<2)
 
 /*
- * Does 'match' match the given name?
- * A match is found if
+ * Does the given pathspec match the given name?  A match is found if
  *
- * (1) the 'match' string is leading directory of 'name', or
- * (2) the 'match' string is a wildcard and matches 'name', or
- * (3) the 'match' string is exactly the same as 'name'.
+ * (1) the pathspec string is leading directory of 'name' ("RECURSIVELY"), or
+ * (2) the pathspec string has a leading part matching 'name' ("LEADING"), or
+ * (3) the pathspec string is a wildcard and matches 'name' ("WILDCARD"), or
+ * (4) the pathspec string is exactly the same as 'name' ("EXACT").
  *
- * and the return value tells which case it was.
+ * Return value tells which case it was (1-4), or 0 when there is no match.
  *
- * It returns 0 when there is no match.
+ * It may be instructive to look at a small table of concrete examples
+ * to understand the differences between 1, 2, and 4:
+ *
+ *                              Pathspecs
+ *                |    a/b    |   a/b/    |   a/b/c
+ *          ------+-----------+-----------+------------
+ *          a/b   |  EXACT    |  EXACT[1] | LEADING[2]
+ *  Names   a/b/  | RECURSIVE |   EXACT   | LEADING[2]
+ *          a/b/c | RECURSIVE | RECURSIVE |   EXACT
+ *
+ * [1] Only if DO_MATCH_DIRECTORY is passed; otherwise, this is NOT a match.
+ * [2] Only if DO_MATCH_LEADING_PATHSPEC is passed; otherwise, not a match.
  */
 static int match_pathspec_item(const struct index_state *istate,
 			       const struct pathspec_item *item, int prefix,
@@ -353,7 +364,7 @@ static int match_pathspec_item(const struct index_state *istate,
 			 item->nowildcard_len - prefix))
 		return MATCHED_FNMATCH;
 
-	/* Perform checks to see if "name" is a super set of the pathspec */
+	/* Perform checks to see if "name" is a leading string of the pathspec */
 	if (flags & DO_MATCH_LEADING_PATHSPEC) {
 		/* name is a literal prefix of the pathspec */
 		int offset = name[namelen-1] == '/' ? 1 : 0;