FindGit: Avoid finding VS 2017 non-general Git installation

At a VS 2017 command prompt the `PATH` contains a directory that happens
to have a `Git/cmd/git.exe` inside it.  However, this executable is not
meant for general use.  Revise our use of `Git/` path suffixes to be
more specific to its original purpose of searching in the typical
`c:/Program Files*/Git/` installation directories.  Avoid using the
suffixes on `PATH` entries.

Fixes: #16706