rebase: support -X to pass through strategy options

rebase: support -X to pass through strategy options

git-rebase calls out to merge strategies, but did not support merge
strategy options so far.  Add this, in the same style used in
git-merge.

Sadly we have to do the full quoting/eval dance here, since
merge-recursive supports the --subtree=<path> option which potentially
contains whitespace.

This patch does not cover git rebase -i, which does not call any merge
strategy directly except in --preserve-merges, and even then only for
merges.

[jc: with a trivial fix-up for 'expr']

Signed-off-by: Mike Lundy <mike@fluffypenguin.org>
Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>