rpm-ch: make it possible to commit/tag the changes

rpm-ch: make it possible to commit/tag the changes

Implement '--tag' command line option (and other related options for
signing) for creating and tagging a release. These correspond the
tagging options in git-buildpackage-rpm.

The git-buildpackage-rpm tool does not commit anything to git. However,
in rpm-ch the '--tag' option causes the changelog modifications (and,
all other staged changes) to be committed to git before creating the
tag. This makes it possible to create a release and document the
packaging/release tag name in the rpm changelog.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>