git-reflog: add option --rewrite to update reflog entries while expiring

git-reflog: add option --rewrite to update reflog entries while expiring

Certain sanity checks on the reflog assume that each entry will contain
a reference to the previous entry. i.e. that the "old" sha1 field of a
reflog entry will be equal to the "new" sha1 field of the previous entry.

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 rewrite the "old" sha1 field of each reflog entry so that it
points to the previous reflog entry.

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