Branches for development

[Man Cao]

I'd vote to be consistent with other projects such as Loom or Valhalla,
that "default" is the mirror of jdk/jdk.
Being consistent would at least avoid confusion when contributors from
other projects participate in the tsan project. And there might be a
technical reason that we are unaware of, e.g., using "default" as the
mirror might ease the merge process somehow.
Is there a reason why "mirror" as the mirror and "default" as the
development branch is superior to the other way round?

For "stable" and "unstable", it might be better that we start off with just
one development branch ("dev" or "tsan" are better names than "unstable",
IMO), and when there's a need to demonstrate something, we fork a
"stable-[timestamp]" branch if we feel it's necessary.
Maintaining two ongoing branches requires extra work for merging and
porting, which is likely troublesome.

-Man