Merge branch 'cb/open-noatime-clear-errno' into maint

Merge branch 'cb/open-noatime-clear-errno' into maint

When trying to see that an object does not exist, a state errno
leaked from our "first try to open a packfile with O_NOATIME and
then if it fails retry without it" logic on a system that refuses
O_NOATIME.  This confused us and caused us to die, saying that the
packfile is unreadable, when we should have just reported that the
object does not exist in that packfile to the caller.

* cb/open-noatime-clear-errno:
  git_open_noatime: return with errno=0 on success