Add get-column and get-data. Use them for faster incremental db update.
add-r-commits now adds nil relation to unreadable project.clj's

Without this the missing relation was interpreted as unread project.clj,
triggering re-reading and flagging the db as dirty. It would also likely cause
problems for our core.logic queries.
Add 'updated-git-db to easily get correct db

Uses new 'read-git-db, updates incrementally as needed from voom-repos, and
writes using 'write-git-db if needed.  Db file is stored in .git dir, contains
a versioning header, and is gzipped.