Rename git-rebase interactive buffer: todo => git-rebase-todo

When using emacsclient or similar, a temporary buffer (file) named
'todo' could cause confusion with a pre-existing buffer of the same
name.

Signed-off-by: Seth Falcon <sethfalcon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -17,7 +17,7 @@ USAGE='(--continue | --abort | --skip | [--preserve-merges] [--verbose]
 require_work_tree
 
 DOTEST="$GIT_DIR/.dotest-merge"
-TODO="$DOTEST"/todo
+TODO="$DOTEST"/git-rebase-todo
 DONE="$DOTEST"/done
 MSG="$DOTEST"/message
 SQUASH_MSG="$DOTEST"/message-squash