Enable p4-fusion by default

[ryanslade]

At this point p4-fusion has feature parity with git-p4. We should move p4-fusion out of experimental mode and make it our default.

• [ ] Mark git-p4 in as deprecated
• [ ] Update docs to make p4-fusion the recommended path
• [ ] Figure our migration path from git-p4 to p4-fusion
  • [ ] Config schema will need to change
  • [ ] Investigate whether conversion that first used git-p4 can switch to p4-fusion without the need to delete the repo in disk and start again.
  • [ ] If the above does not work, we may need an OOB migration to work through old repos
• [ ] After a few releases, make p4-fusion default in config and remove support for git-p4