cindex: rename --associate to --join, test w/ real repos

cindex: rename --associate to --join, test w/ real repos

The association data is just stored as deflated JSON in Xapian
metadata keys of shard[0] for now.  It should be reasonably
compact and fit in memory for now since we'll assume sane,
non-malicious git coderepo history, for now.

The new cindex-join.t test requires TEST_REMOTE_JOIN=1 to be
set in the environment and tests the joins against the inboxes
and coderepos of two small projects with a common history.

Internally, we'll use `ibx_off', `root_off' instead of `ibx_id'
and `root_id' since `_id' may be mistaken for columns in an SQL
database which they are not.