Git revision information broke compatibility of `--version` output

[geekosaur]

Describe the bug
When there is no git revision information, a spurious "(in-tree)" is being output in the Cabal library version. This includes for released versions of cabal-install.

To Reproduce
Steps to reproduce the behavior:

$ cabal --version

Expected behavior
The output should be compatible with earlier cabal-install versions.

System information

• cabal-install 3.16.0.0