Fix non-fast-forward detection to use is_infix, not is_prefix
The git push stderr starts with "To /path/..." not with the error
keywords. Use Astring.String.is_infix to search anywhere in stderr.
Also reverted the auto-force and checkout_tree plumbing — the
non-fast-forward on local checkouts is a one-time consequence of
the split skip fix (old split included no-op commits, new split
correctly skips them). Resolved with monopam push --force once.