wrapper.c: add and use warn_on_fopen_errors()

In many places, Git warns about an inaccessible file after a fopen()
failed. To discern these cases from other cases where we want to warn
about inaccessible files, introduce a new helper specifically to test
whether fopen() failed because the current user lacks the permission to
open file in question.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/dir.c b/dir.c
index f451bfa..be616e8 100644
--- a/dir.c
+++ b/dir.c
@@ -745,9 +745,9 @@ static int add_excludes(const char *fname, const char *base, int baselen,
 
 	fd = open(fname, O_RDONLY);
 	if (fd < 0 || fstat(fd, &st) < 0) {
-		if (errno != ENOENT)
-			warn_on_inaccessible(fname);
-		if (0 <= fd)
+		if (fd < 0)
+			warn_on_fopen_errors(fname);
+		else
 			close(fd);
 		if (!check_index ||
 		    (buf = read_skip_worktree_file_from_index(fname, &size, sha1_stat)) == NULL)