Polybar picks up wrong version number when release tarball is built under foreign repo

### Steps to reproduce



1. Download a [polybar release](https://github.com/polybar/polybar/releases)

2. Extract it in a nested folder of *another* git repo (that repo has to have at least one tag on the current branch)

3. Build polybar regularly



### Expected behavior



Since polybar is not built from its git repo, it should extract the version information from the `version.txt` file



### Actual behavior



The `git describe ...` command used to determine the version (and whether we are in a git repo) picks up the version tag from the parent repo instead.



### Additional Context / Screenshots



The top-level `CMakeLists.txt` should additionally check that the top-level git directory matches the top-level cmake directory. This way, we only use git to determine the version if the source code is in the top-level of the repo.



This still may get "wrong" results if someone is just putting the polybar sources at the top of some other repo. But that case is not distinguishable from polybar forks anyway and is acceptable.



**PRs are welcome**

[]