RebaseCommand: fix ONTO_NAME, and --preserve-merges is interactive

RebaseCommand: fix ONTO_NAME, and --preserve-merges is interactive

ONTO_NAME must be "onto_name", not "onto-name".

For native git, --preserve-merges is an interactive mode. Create the
INTERACTIVE marker file, otherwise a native git rebase --continue
will fall back into rebase --merge mode before git 2.19.0 since it
started looking for the REWRITTEN directory to make the distinction
only then.[1]

This allows a JGit interactive rebase to be continued via native git
rebase --continue.

[1] https://github.com/git/git/commit/6d98d0c0

Bug: 511487
Change-Id: I13850e0fd96ac77d03fbb581c8790d76648dbbc6
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>