R1:9d5a2b3b4f68

Add a RefPosition table to hold branch/tag positions once the RefCursor table is split

Summary:
Ref T11823. Currently, we have a "RefCursor" table which stores rows like `<branch or tag name, commit it is pointing at>` with some more data.

Because Mercurial can have a single branch pointing at several different places, this table must allow multiple rows with the same branch or tag name.

Among other things, this means there isn't a single PHID which can be used to identify a branch name in a stable way.