git-rebase: make 'rebase HEAD branch' work as expected.

git-rebase: make 'rebase HEAD branch' work as expected.

When you want to amend the commit message of 3 commits before
the tip of the current branch, say 'master',

	A--B--C--D--E(master)

it is sometimes handy to make your head detached at that commit
with:

	$ git checkout HEAD~3 ;# check out B
	$ git commit --amend ;# without modifying contents...

to create:

          .B'(HEAD)
         /
	A--B--C--D--E(master)

and then rebase 'master' branch onto HEAD with this:

	$ git rebase HEAD master

to result in:

          .B'-C'-D'-E(master=HEAD)
         /
	A--B--C--D--E

However, the current code interprets HEAD after it switches to
the branch 'master', which means the rebase will not do
anything.  You have to say something unwieldly like this
instead:

	$ git rebase $(git rev-parse HEAD) master

This fixes it by expanding the $onto commit name before
switching to the target branch.

Signed-off-by: Junio C Hamano <junkio@cox.net>