add @TAG_OFFSET@ support in versionformat for git

Mark the commit count since @PARENT_TAG@, if there is no @PARENT_TAG@, will count since first commit.

This patch if a python fork of https://github.com/openSUSE/obs-service-tar_scm/pull/26.

If merged, some special version format could be defined in OBS, like this https://github.com/openSUSE/obs-service-tar_scm/issues/55.