Tracking v7 progress toward becoming the default.

## step 1: release

done

## step 2: default for new direct mode repositories

Done when using the command line to init in a crippled filesystem. 

The assistant still creates direct mode repositories though,
and will need to enable annex.thin and adjusted unlocked branch instead.

## step 3: auto-upgrade from direct mode

Direct mode is very buggy and limited, so it's easy for v7 (with adjusted
unlocked branches) to be better than it.

Need to give it some time for problems to be found though.

Here are some problems that need to be investigated first:

  - <http://git-annex.branchable.com/bugs/assistant_crashes_in_TransferScanner/>
  - <http://git-annex.branchable.com/bugs/v6_appears_to_not_thin/>
  - <http://git-annex.branchable.com/bugs/git-annex-sync_sometimes_fails_in_submodule_in_V6_adjusted_branch/>

Note that direct mode repos with old git-annex interoperate with adjusted
unlocked repos with new git-annex, so there is no need to wait for v7 to be
widely supported.

## step 4: default for all new repositories

Could probably happen fairly soon after default in direct mode.
This is entirely new repositories that git-annex init is run in for the
first time (no sibling git-annex branches).

## step 5: automatic v5 to v7 upgrades

Since v5 repos and v7 repos not using unlocked files are functionally
almost identical, this is unlikely to break much. Unlocking files will of
course change behavior though.

This needs to be deferred until every git-annex likely to be in use
supports v7. A v5 repository may be used by multiple
machines, some not supporting v7 and some that do. If one upgrades to v7
and starts using unlocked files, those files won't be accessible on the old
v5 machines.

## step 6: remove support for direct mode

Will simplify a lot of code.

## step 7: remove support for v5

This won't simplify much code, but maybe worth doing eventually.
Best not to rush this, maybe in 5 years from when v7 is default?