git_open_noatime: return with errno=0 on success

In read_sha1_file_extended we die if read_object fails with a fatal error. We detect a fatal error if errno is non-zero and is not ENOENT. If the object could not be read because it does not exist, this is not considered a fatal error and we want to return NULL. Somewhere down the line, read_object calls git_open_noatime to open a pack index file, for example. We first try open with O_NOATIME. If O_NOATIME fails with EPERM, we retry without O_NOATIME. When the second open succeeds, errno is however still set to EPERM from the first attempt. When we finally determine that the object does not exist, read_object returns NULL and read_sha1_file_extended dies with a fatal error: fatal: failed to read object <sha1>: Operation not permitted Fix this by resetting errno to zero before we call open again. Cc: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Clemens Buchacher <clemens.buchacher@intel.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>