Allow overwriting version using `.git/versioningit-pretend-version`

[basnijholt]

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Previously I opened https://github.com/jwodder/versioningit/pull/76 which was closed.

I (and my team) keep running into the same issue where we need to copy in a huge .git folder just to make our package installable. In a mono repo with hundreds of installable packages this is certainly not desirable.

This PR allows to overwrite the version via a simple file.

xref: https://github.com/jwodder/versioningit/issues/45#issuecomment-2427839822