dir: fix COLLECT_IGNORED on excluded prefixes

As we walk the directory tree, if we see an ignored path, we
want to add it to the ignored list only if it matches any
pathspec that we were given. We used to check for the
pathspec to appear explicitly. E.g., if we see "subdir/file"
and it is excluded, we check to see if we have "subdir/file"
in our pathspec.

However, this interacts badly with the optimization to avoid
recursing into ignored subdirectories. If "subdir" as a
whole is ignored, then we never recurse, and consider only
whether "subdir" itself is in our pathspec.  It would not
match a pathspec of "subdir/file" explicitly, even though it
is the reason that subdir/file would be excluded.

This manifests itself to the user as "git add subdir/file"
failing to correctly note that the pathspec was ignored.

This patch extends the in_pathspec logic to include prefix
directory case.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

dir.c

@@ -554,13 +554,29 @@ static int simplify_away(const char *path, int pathlen, const struct path_simpli
 	return 0;
 }
 
-static int in_pathspec(const char *path, int len, const struct path_simplify *simplify)
+/*
+ * This function tells us whether an excluded path matches a
+ * list of "interesting" pathspecs. That is, whether a path matched
+ * by any of the pathspecs could possibly be ignored by excluding
+ * the specified path. This can happen if:
+ *
+ *   1. the path is mentioned explicitly in the pathspec
+ *
+ *   2. the path is a directory prefix of some element in the
+ *      pathspec
+ */
+static int exclude_matches_pathspec(const char *path, int len,
+		const struct path_simplify *simplify)
 {
 	if (simplify) {
 		for (; simplify->path; simplify++) {
 			if (len == simplify->len
 			    && !memcmp(path, simplify->path, len))
 				return 1;
+			if (len < simplify->len
+			    && simplify->path[len] == '/'
+			    && !memcmp(path, simplify->path, len))
+				return 1;
 		}
 	}
 	return 0;
@@ -638,7 +654,7 @@ static enum path_treatment treat_one_path(struct dir_struct *dir,
 {
 	int exclude = excluded(dir, path, &dtype);
 	if (exclude && (dir->flags & DIR_COLLECT_IGNORED)
-	    && in_pathspec(path, *len, simplify))
+	    && exclude_matches_pathspec(path, *len, simplify))
 		dir_add_ignored(dir, path, *len);
 
 	/*