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

This updates BUILD_VERSION to store a string of the form `<sha1>` representing the HEAD commit checked out in the workspace. That string is the fourth part of the version number, when it is available.

```bash
$ chpl --version
chpl Version 1.9.0.c403259
Copyright (c) 2004-2014, Cray Inc.  (See LICENSE file for more details)
```
This commit does not change behavior of when BUILD_VERSION is generated.

Eventually we may consider adding something like the commit date, so it is easier for humans to reason about the relative version. That requires additional discussion, so it probably makes sense to use this for now.

### TODO

* [x] run testing against full suite
* [x] verify --version works on different platforms
* [x] verify --version works from release tarball (should not include fourth version number)
Any reason to remove this line?  Cleanliness?