finalize_object_file(): refactor unlink_or_warn() placement

finalize_object_file(): refactor unlink_or_warn() placement

As soon as we've tried to link() a temporary object into place, we then
unlink() the tempfile immediately, whether we were successful or not.

For the success case, this is because we no longer need the old file
(it's now linked into place).

For the error case, there are two outcomes. Either we got EEXIST, in
which case we consider the collision to be a noop. Or we got a system
error, in which we case we are just cleaning up after ourselves.

Using a single line for all of these cases has some problems:

  - in the error case, our unlink() may clobber errno, which we use in
    the error message

  - for the collision case, there's a FIXME that indicates we should do
    a collision check. In preparation for implementing that, we'll need
    to actually hold on to the file.

Split these three cases into their own calls to unlink_or_warn(). This
is more verbose, but lets us do the right thing in each case.

Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>