Conflicts exist if move to jdk14

[Man Cao]

I also looked at how other projects (portola and shenandoah) handle this.
Unlike tsan and loom, which maintain separate development branches,
both portola and shenandoah just maintain a single "default" branch. They
also have separate repositories for JDK11, JDK12, etc.
They also do regular "Merge" commits to sync with jdk/jdk in the master
development repository.
So I think we should go with "hg merge". Mercurial should be able to
differentiate commits between the tsan and jdk/jdk. The "tsan" branch in
our repository would help separate out the changes further.

I guess the main problem with "hg rebase" is that it generates a whole new
set of changeset IDs (hashes). We don't want to change the IDs for
changesets that have already been pushed.

Any more concerns before I try a merge with jdk14-ga?

-Man