buildpackage: new "working copy" choices for --git-export

Add support for building different kind of "working copies", when using
the --git-export option:
- 'WC.TRACKED': only use files that are already tracked
- 'WC.UNTRACKED': use untracked files, too
- 'WC.IGNORED': also add files that'd normally be ignored

Using '--git-export=WC' beaves like before.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Conflicts:
	gbp/scripts/common/buildpackage.py