annex-get files to PUBLIC repo; filter-branch cmds > sh script

- Instead of forcing user to copy-and-paste from terminal, write
  the filter-branch commands
- Keep working data files out in destdir instead of hiding them
  in the TEMP repo.
- Final PUBLIC repo git-annex gets binaries from TEMP repo.
clearer handling of public repo path; temp repo delete now optional

- Generate final public repo path in main() and pass to mk_filterbranch_script() as an arg.
- Make deletion of temporary repo optional via an arg.
- Comments clarification.
functions for running filter-branch in subprocess (disabled)

I'd like to run filter-branch in a subprocess so the user doesn't
have to run a second shell script, but couldn't get it to work.
Functions are included here for reference.
clearer semantics for mk_filterbranch(); exit if filter fails

- Removed filterbranch_ok arg, it's not used outside this function.
- Changed "rm temp repo" to "keep temp repo".
- Added command-line arg for keeping temp repo.
- Better output messaging.
print output to logfile in addition to console