[llvm-env] List current worktree directories

When running llvm-env without any arguments, it now dumps the current worktrees,
as well as the branch that is currently checked out in them (only for llvm, to
make it easy to spot places where this is different from the environment name).

This is a very hacky implementation, because newer versions of git will come
with git worktree list, which does exactly what we want. We can nuke this when
we're all ready to move to newer gits.

Change-Id: I787594ce1ecea0de68ceb0e518194b783da408b3
1 file changed