* Loading LibGit2Sharp.dll by hand in msbuild no longer works in VS2022 due to
architecture changes. We can fairly reliably get the same result by manually
reading (and following the ref in) .git/HEAD
* We also only use GIT_COMMIT_HASH where necessary to avoid rebuilding many
files for no reason, including splitting version.cpp out into a separate
project as we do with VS since otherwise changing its preprocessor defines
rebuilds the whole renderdoc project.
* At the same time, move the git hash to be internal only so we don't have to
try to link version.cpp into other projects like renderdoccmd or qrenderdoc.
* On windows, the change in a global GIT_COMMIT_HASH define in each
project needing it meant a full rebuild every time the commit changed.
* Ideally we'd set the define only on one file in each project, but
MSBuild doesn't seem to support that. Instead we make a new tiny
project that compiles a single cpp exporting a global var, and
reference that global var in each other project.
baldurk