Update chpl --version to use git sha1 and branch, instead of svn revision

This updates BUILD_VERSION to store a string of the form "<branch>@<sha1>"
representing the branch and commit checked out in the workspace. That string is
the fourth part of the version number, when it is available.

This commit does not change behavior of when BUILD_VERSION is generated.

### TODO

* [ ] run testing against full suite
* [ ] verify --version works on different platforms
* [ ] verify --version works from release tarball (should not include fourth version number)