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.
Remove FIXME from updateBuildVersion.
Simplify fourth part of version to just sha1.

The branch name does not add much clarity.
Add linebreak that was removed on accident...
Merge pull request #25 from thomasvandoren/git-sha1-for-build-version

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