R1:6f3a99eb392e

Generate ref updates in Mercurial hooks

Summary: Ref T4195. Mercurial is not my favorite VCS.

Test Plan:
Hit the split branches case:

>>> orbital ~/repos/INIH $ hg push --force
pushing to ssh://dweller@local.aphront.com/diffusion/INIH
searching for changes
remote: adding changesets
remote: adding manifests
remote: adding file changes
remote: added 2 changesets with 2 changes to 1 files (+1 heads)
remote: +---------------------------------------------------------------+
remote: | *…