Fixes many previously unknown issues with git xet repo migrate.
-> Notes were not migrated.
-> Tags were not correctly migrated.
-> Merge and cherry-pick commits were not properly tracked as such. 

The logic to properly do all of this required rethinking the flow. The
new design is MUCH better and more laid out, but is essentially a
rewrite of all of it.

---------

Co-authored-by: Hoyt Koepke <hoytak@xethub.com>