rebase: align variable names

git-rebase--interactive.sh will soon be sourced from
git-rebase.sh. Align the names of variables used in these scripts to
prepare for that.

Some names in git-rebase--interactive.sh, such as "author_script" and
"amend", are currently used in their upper case form to refer to a
file and in their lower case form to refer to something else. In these
cases, change the name of the existing lower case variable and
downcase the name of the variable that refers to the file.

Currently, git-rebase.sh uses mostly lower case variable names, while
git-rebase--interactive.sh uses mostly upper case variable names. For
consistency, downcase all variables, not just the ones that will be
shared between the two script files.

Helped-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>