Don't warn on tag missing when subproject (#653) (#655)
Different solution than proposed in the issue, since that proposed
solution would given unexpected results when an intermediate
(untagged) commit is checked out.
This change simply skips warning about a missing git version when this
is a subproject, and uses the original version calculation logic.
I've also renamed `determine_version` to `determine_version_with_file`.
I'd originally done this in an intermediate version of this PR, but I
think that keeping the renaming is clearer code.
