[FIXED PATCH] Make rebase save ORIG_HEAD if changing current branch

This makes rebase act a little more like merge when working on the current branch. This is particularly useful for `git pull --rebase`

Signed-off-by: Brian Gernhardt <benji@silverinsanity.com>
---

 ARG!  This is what v3 was supposed to be.  I should make sure I am sending in
 the correct patch.  (Tip: run format-patch again after a commit --amend.)  Bad
 weekend for me, apparently trying to do too many things at once.  Sorry for
 all the noise.

 To recap:

 If I followed the discussion correctly the first time I sent in this patch,
 the two issues were:

 - Ugly "echo > ORIG_HEAD" instead of pretty "git update-ref ORIG_HEAD"
 - Setting ORIG_HEAD at the wrong place

 This version (as opposed to v2, or the embarrassingly identital "v3")  uses
 the correct variable.  $orig_head looks like the right name, but it stores the
 branch.  $prev_head stores the actual SHA1, which is what I was looking for.

 git-rebase.sh |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)