buildpackage-rpm: add option --git-ignore-untracked

Adds a new commandline option to ignore untracked files, when running
git-buildpackage-rpm. When this option is given, the
git-buildpackage-rpm command fails if there are changes to tracked
files, but, succeeds if there are untracked files present. Normally it
fails in either case.

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