rebase -i: align variable names

rebase -i: align variable names

Rename variables HEAD and OLDHEAD to orig_head and HEADNAME to
head_name, which are the names used in git-rebase.sh. This prepares
for factoring out of the code that persists these variables during the
entire rebase process. Using the same variable names to mean the same
thing in both files also makes the code easier to read.

While at it, also remove the DOTEST variable and use the state_dir
variable that was inherited from git-rebase.sh instead.

Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>