Always ensure the pack.keep file is removed by git-fetch

Always ensure the pack.keep file is removed by git-fetch

If we are using a native transport and the transport chose to
save the packfile it may have created a .keep file to protect
the packfile from a concurrently running git-repack process.

In such a case the git-fetch process should make sure it will
unlink the .keep file even if it fails to update any refs as
otherwise the newly downloaded packfile's diskspace will never
be reclaimed if the objects are not actually referenced.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>