clean: use warning_errno() when appropriate

All these warning() calls are preceded by a system call. Report the
actual error to help the user understand why we fail to remove
something.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/clean.c

@@ -174,8 +174,10 @@ static int remove_dirs(struct strbuf *path, const char *prefix, int force_flag,
 		/* an empty dir could be removed even if it is unreadble */
 		res = dry_run ? 0 : rmdir(path->buf);
 		if (res) {
+			int saved_errno = errno;
 			quote_path_relative(path->buf, prefix, &quoted);
-			warning(_(msg_warn_remove_failed), quoted.buf);
+			errno = saved_errno;
+			warning_errno(_(msg_warn_remove_failed), quoted.buf);
 			*dir_gone = 0;
 		}
 		return res;
@@ -208,8 +210,10 @@ static int remove_dirs(struct strbuf *path, const char *prefix, int force_flag,
 				quote_path_relative(path->buf, prefix, &quoted);
 				string_list_append(&dels, quoted.buf);
 			} else {
+				int saved_errno = errno;
 				quote_path_relative(path->buf, prefix, &quoted);
-				warning(_(msg_warn_remove_failed), quoted.buf);
+				errno = saved_errno;
+				warning_errno(_(msg_warn_remove_failed), quoted.buf);
 				*dir_gone = 0;
 				ret = 1;
 			}
@@ -230,8 +234,10 @@ static int remove_dirs(struct strbuf *path, const char *prefix, int force_flag,
 		if (!res)
 			*dir_gone = 1;
 		else {
+			int saved_errno = errno;
 			quote_path_relative(path->buf, prefix, &quoted);
-			warning(_(msg_warn_remove_failed), quoted.buf);
+			errno = saved_errno;
+			warning_errno(_(msg_warn_remove_failed), quoted.buf);
 			*dir_gone = 0;
 			ret = 1;
 		}
@@ -981,8 +987,10 @@ int cmd_clean(int argc, const char **argv, const char *prefix)
 		} else {
 			res = dry_run ? 0 : unlink(abs_path.buf);
 			if (res) {
+				int saved_errno = errno;
 				qname = quote_path_relative(item->string, NULL, &buf);
-				warning(_(msg_warn_remove_failed), qname);
+				errno = saved_errno;
+				warning_errno(_(msg_warn_remove_failed), qname);
 				errors++;
 			} else if (!quiet) {
 				qname = quote_path_relative(item->string, NULL, &buf);