catacomb-import-update: Separate import from convert

catacomb-import-update: Separate import from convert

We want the actual source (PFM) in the secnet git repository, so we
should include the *un*converted files, and convert at build time.

So program is now to be called twice: --import, to update the local
git repository from ../catacomb into ./catacomb-import, and --convert,
to massage those into the output files we want for secnet.

We have a new variable $DOCONVERT which says which mode we're working
in.  It is most convenient to test this in note_path and convert_*,
rather than at each call site.

The only nontrivial changes are to commit_changes, where we no longer
care about recip paths and just use donor paths.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>