repo: implement repo as a master driven source step

This commit reimplements the same feature and API
as repo from oldsource.py
The big difference is the test coverage that is now 98%,
and thus the maintainability of the code.
Most of the code is also now written in inline callbacks which adds
a lot of readability.

Additionnal features compared to oldsource.py:

- repo is now capable of importing patches to the manifest directory
  This allows for gated-commit process to also verify that manifest patches
  does not break the build
- better handling of tarball (automatic update on a weekly basis)
- support for repo sync -c, which speeds up a lot in very big projects
- better clobbering behaviour. The working directory is kept as much as
  possible for obvious speed reasons

Version2:
- reworked the flow to use less unneeded methods (like continueVC)
- reworked setText for concision
- reworked manifest patch handling which did not actually work with repo
- also remove the lock for manifests repository

Version3:
- fix bug where tarball is not created the first time (+added a test for that)
- log the environ only once (and factorize it in tests)

Version4:
- Only encode manifest branches when needed
- make mirror retry to really be a retry number, and not a try number

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>