Remove pack.keep after ref updates in git-fetch

Remove pack.keep after ref updates in git-fetch

If we are using a native packfile to perform a git-fetch invocation
and the received packfile contained more than the configured limits
of fetch.unpackLimit/transfer.unpackLimit then index-pack will output
a single line saying "keep\t$sha1\n" to stdout.  This line needs to
be captured and retained so we can delete the corresponding .keep
file ("$GIT_DIR/objects/pack/pack-$sha1.keep") once all refs have
been safely updated.

This trick has long been in use with git-fetch.sh and its lower level
helper git-fetch--tool as a way to allow index-pack to save the new
packfile before the refs have been updated and yet avoid a race with
any concurrently running git-repack process.  It was unfortunately
lost when git-fetch.sh was converted to pure C and fetch--tool was
no longer being invoked.

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