Use it from ExternalProject and the ExternalProject test's
CMakeLists file rather than having duplicate find_program calls.
Add logic so that we do not try to use *.cmd variants of git
programs when using the MSYS Makefiles generator. Should fix
the last remaining dashboard issue with the new ExternalProject
git support additions.

Also, correct minor problem regarding placement of the local git
repo during test execution. On clean builds, it was being placed
incorrectly because of the ../.. relative reference. Use an absolute
path to place the local git repo in the proper directory, and only
use the relative reference when referring to it.