add @SEQUENCE_NUMBER@ support in versionformat for git

Mark the commit count since @PARENT_TAG@, if there
is no @PARENT_TAG@, will count since first commit.

Well, previous pull request was closed incorrect by me, so another one is here.
Sorry if I was unclear, but I was only talking about the tag-less case.  When there is a tag, `@TAG_OFFSET@` is better, since there is effectively a sequence per tag so `@SEQUENCE_NUMBER@` has a confusing meaning.