buildpackage: add 'untracked' option to write_wc()

Whether to ignore untracked files (untracked=False) or not
(untracked=True). Now, clones the temporary index file from the actual
index, in order to correctly update files.

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