Blender Git Loki

CMake: Avoid call of find_package(Git) for every compile with buildinfo enabled

This is a bit tricky, find_package(Git) was only used to check if git is installed
and actual git command was assumed to be in the PATH (while ideally it should have
been GIT_COMMAND variable).

This commit makes re-compile output cleaner, especially when using Ninja.

Adding proper GIT_COMMAND to buildinfo.cmake is also possible via argument.

Reviewers: campbellbarton

Reviewed By: campbellbarton

Differential Revision: https://developer.blender.org/D1847