plugins/git.py: Fail if ref is not in given track

Add a helper function assert_ref_in_track to git.py GitMirror()
which is used when staging & initing the source workspace. It
checks the element's ref exists in the track (branch/tag) if it
has been specified, raising an error if necessary. If the element
has been tracked with bst, it is assumed that the value of ref exists
in the track as it was generated from it & as such is not asserted.

WIP: Fix for issue #483