lit: Add a test for discovery when exact test names are given.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179247 91177308-0d34-0410-b5e6-96231b3b80d8
lit: Shorten a metavar to make --help look nicer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179248 91177308-0d34-0410-b5e6-96231b3b80d8
lit: Don't descend into .git directories during test discovery.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179249 91177308-0d34-0410-b5e6-96231b3b80d8