fix cherry-picking routine

Tolerate only those errors where cherry-pick
exited with 1. Other issues -- like failing on
not wanting to overwrite untracked files --
should not be glossed over.
keep tree clean between checkouts with 'git rm'

'git update-index --force-remove', that we used
so far, affects only the index. git rm also deletes
the working directory copies avoiding cruft in
working dir, too.

Also take care to find out the exact set of unneeded
files as we don't want to do unneeded changes in the
working dir.