Make build-script agnostic of local git installation

[Vanmodeus]

Projects in the HeuristicLab solution generate AssemblyInfo.cs and Plugin.cs classes during compile-time (based on their respective .cs.frame files). During this pre-build event the version is adapted by replacing a placeholder (e.g. AssemblyFileVersion("3.3.16.$WCREV$")). Previously the external SubWCRev command (requires TortoiseSVN) was utilized for this purpose. Since the migration to GitHub prevents using SubWCRev, this command has been replaced with "git rev-list --count HEAD".
Hence, the build script currently depends on the git command and requires a local installation of git (and a respective entry in PATH). We should think about using a fallback logic in the case of a missing git installation and reduce the dependency of the build script on the local environment.

This fix requires an entry of the windows git installation in your PATH variable.

Depending on your git client this might not be done automatically
(e.g. Fork uses a local installation hidden in AppData/Local...).
Without a linked git installation you receive the following error message when building:
<HeuristicLabFolder>\HeuristicLab\3.3\Properties\AssemblyInfo.cs(48,28): error CS7034: The specified version string does not conform to the required format - major[.minor[.build[.revision]]] [D:\src\HeuristicLab\HeuristicLab\3.3\HeuristicL ab-3.3.csproj]

Originally posted by @florianBachinger in #3150 (comment)