unpack-trees: handle lstat failure for existing directory

unpack-trees: handle lstat failure for existing directory

When check_leading_path notices no file in the way of the new entry to
be checked out, verify_absent checks whether there is a directory
there or nothing at all.  If that lstat call fails (for example due to
ENOMEM), it assumes ENOENT, meaning a directory with untracked files
would be clobbered in that case.

Check errno after calling lstat, and for conditions other than ENOENT,
just error out.

This is a theoretical race condition.  lstat has to succeed moments
before it fails for there to be trouble.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>