This fixes a bug where a re-import of the contacts directory failed
because some entries already exist.
With this change, one can import the contact directory after things
changed in there and the imag database is updated accordingly, but not
altered. So links and notes are persisting.
For completeness: before, the workflow was to
`cd $IMAG_RTP && git rm store/contacts -r && imag contact import /path/to/contacts`
which is of course not optimal.
Signed-off-by: Matthias Beyer <mail@beyermatthias.de>