Add Tag Creation for Automatic Labels

[groboclown]

As Perforce supports automatic labels, which are a view mapping + a revision marker, it's possible to map these to a Git tag under very limited criteria:

1. The Label contains the field Revision with a value of @(integer)
2. The Label contains a View with a path that fully contains the depot path being copied.
3. The (integer) revision is a changelist that the transfer includes.

Because this label fully matches to a Git commit, a corresponding tag can be created off of the label.