[vector] Transition from hg to git

[Paul Sandoz]

Hi,

The Vector API work has transitioned from hg to git at this location:

  https://github.com/openjdk/panama-vector <https://github.com/openjdk/panama-vector>

- I have requested that the hg repo transition to read-only. In the interim please do not push to it.
 
- the same branch names are used for development

- the repo will be configured so that the master is automatically mirrored with jdk:master
  master should never be pushed to directly
  Manual merging will still be required from master to other branches.

- I propose we remove many of the stale branches (related to panama-foreign) and change the default branch to vectorIntrinsics

Paul.