git-reflog: add option --updateref to write the last reflog sha1 into the ref

git-reflog: add option --updateref to write the last reflog sha1 into the ref

Certain sanity checks on the reflog assume that the sha1 of the top reflog
entry will be equal to the sha1 stored in the ref.

When reflog entries are deleted, this assumption may not hold. This patch
adds a new option to git-reflog which causes the subcommands "expire" and
"delete" to update the ref with the sha1 of the top-most reflog entry.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>