gitignore: report access errors of exclude files

When we try to access gitignore files, we check for their
existence with a call to "access". We silently ignore
missing files. However, if a file is not readable, this may
be a configuration error; let's warn the user.

For $GIT_DIR/info/excludes or core.excludesfile, we can just
use access_or_warn. However, for per-directory files we
actually try to open them, so we must add a custom warning.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

dir.c

@@ -397,6 +397,8 @@ int add_excludes_from_file_to_list(const char *fname,
 
 	fd = open(fname, O_RDONLY);
 	if (fd < 0 || fstat(fd, &st) < 0) {
+		if (errno != ENOENT)
+			warning(_("unable to access '%s': %s"), fname, strerror(errno));
 		if (0 <= fd)
 			close(fd);
 		if (!check_index ||
@@ -1311,9 +1313,9 @@ void setup_standard_excludes(struct dir_struct *dir)
 		home_config_paths(NULL, &xdg_path, "ignore");
 		excludes_file = xdg_path;
 	}
-	if (!access(path, R_OK))
+	if (!access_or_warn(path, R_OK))
 		add_excludes_from_file(dir, path);
-	if (excludes_file && !access(excludes_file, R_OK))
+	if (excludes_file && !access_or_warn(excludes_file, R_OK))
 		add_excludes_from_file(dir, excludes_file);
 }