plugins/git.py: Warn 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 a warning if necessary. The warning makes
use of the warning token 'REF_NOT_IN_TRACK' from the configurable
CoreWarnings. 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.