unpack-trees: rename 'is_excluded_from_list()'

The first consumer of pattern-matching filenames was the
.gitignore feature. In that context, storing a list of patterns
as a 'struct exclude_list'  makes sense. However, the
sparse-checkout feature then adopted these structures and methods,
but with the opposite meaning: these patterns match the files
that should be included!

Now that this library is renamed to use 'struct pattern_list'
and 'struct pattern', we can now rename the method used by
the sparse-checkout feature to determine which paths should
appear in the working directory.

The method is_excluded_from_list() is only used by the
sparse-checkout logic in unpack-trees and list-objects-filter.
The confusing part is that it returned 1 for "excluded" (i.e.
it matches the list of exclusions) but that really manes that
the path matched the list of patterns for _inclusion_ in the
working directory.

Rename the method to be path_matches_pattern_list() and have
it return an explicit 'enum pattern_match_result'. Here, the
values MATCHED = 1, UNMATCHED = 0, and UNDECIDED = -1 agree
with the previous integer values. This shift allows future
consumers to better understand what the retur values mean,
and provides more type checking for handling those values.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

dir.c

@@ -1072,19 +1072,28 @@ static struct path_pattern *last_matching_pattern_from_list(const char *pathname
 }
 
 /*
- * Scan the list and let the last match determine the fate.
- * Return 1 for exclude, 0 for include and -1 for undecided.
+ * Scan the list of patterns to determine if the ordered list
+ * of patterns matches on 'pathname'.
+ *
+ * Return 1 for a match, 0 for not matched and -1 for undecided.
  */
-int is_excluded_from_list(const char *pathname,
-			  int pathlen, const char *basename, int *dtype,
-			  struct pattern_list *pl, struct index_state *istate)
+enum pattern_match_result path_matches_pattern_list(
+				const char *pathname, int pathlen,
+				const char *basename, int *dtype,
+				struct pattern_list *pl,
+				struct index_state *istate)
 {
 	struct path_pattern *pattern;
 	pattern = last_matching_pattern_from_list(pathname, pathlen, basename,
 						  dtype, pl, istate);
-	if (pattern)
-		return pattern->flags & PATTERN_FLAG_NEGATIVE ? 0 : 1;
-	return -1; /* undecided */
+	if (pattern) {
+		if (pattern->flags & PATTERN_FLAG_NEGATIVE)
+			return NOT_MATCHED;
+		else
+			return MATCHED;
+	}
+
+	return UNDECIDED;
 }
 
 static struct path_pattern *last_matching_pattern_from_lists(