object-file: don't special-case missing source file in collision check

object-file: don't special-case missing source file in collision check

In 0ad3d65652 (object-file: fix race in object collision check,
2024-12-30) we have started to ignore ENOENT when opening either the
source or destination file of the collision check. This was done to
handle races more gracefully in case either of the potentially-colliding
disappears.

The fix is overly broad though: while the destination file may indeed
vanish racily, this shouldn't ever happen for the source file, which is
a temporary object file (either loose or in packfile format) that we
have just created. So if any concurrent process would have removed that
temporary file it would indicate an actual issue.

Stop treating ENOENT specially for the source file so that we always
bubble up this error.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>