rebase: remove unused rebase state 'prev_head'
rebase: remove unused rebase state 'prev_head'

The state stored in $GIT_DIR/rebase-merge/prev_head was introduced in
58634db (rebase: Allow merge strategies to be used when rebasing,
2006-06-21), but it was never used and should therefore be removed.

Signed-off-by: Martin von Zweigbergk <4a0689660d2c85c65ee5252dabed282f8fad3a91@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git