Remove GetGitRevisionDescription.cmake

[Ereski]

GetGitRevisionDescription.cmake uses a somewhat contrived method to obtain the
hash of the commit pointed by the repository's HEAD. This method fails on
unusual, but still valid repository layouts.

This commit removes cmake/GetGitRevisionDescription.cmake{,.in}, replacing its
functionality with direct use of Git's plumbing.

[zinnschlag]

I don't understand what problem this change is supposed to fix.

This method fails on
unusual, but still valid repository layouts.

Okay, but it works with the OpenMW repository layout, right? So we actually do not need to change anything.

[Ereski]

Sorry, bad wording. It's the layout of the .git directory. It fails if it is not actually a directory (.git can actually be a file), or if the refs are packed and .git/logs/refs/heads/<branch> doesn't exist. In other words, it tries to get the information in a completely ad-hoc way, failing on valid repositories that deviate from the author's expectations. Case in point: it did fail on my machine.

OTOH, Git already knows how to handle every corner case.