am --abort: keep unrelated commits on unborn branch

am --abort: keep unrelated commits on unborn branch

Since 7b3b7e3 (am --abort: keep unrelated commits since the last failure
and warn, 2010-12-21), git-am would refuse to rewind HEAD if commits
were made since the last git-am failure. This check was implemented in
safe_to_abort(), which checked to see if HEAD's hash matched the
abort-safety file.

However, this check was skipped if the abort-safety file was empty,
which can happen if git-am failed while on an unborn branch. As such, if
any commits were made since then, they would be discarded. Fix this by
carrying on the abort safety check even if the abort-safety file is
empty.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>