BUG: ExtensionBuildSystem: Ensure GIT_EXECUTABLE set in SliceConfig is used

This problem is not revealed when Slicer extensions are
configured and built automatically by the Extensions build system setup
on the factory machines. In that case, variables like git and svn paths
are explicitly passed to the extension and added to the cache
using the "-D" cmake arguments.

Instead, this commit fixes a problem occurring when an extension is
configured manually by only passing "-DSlicer_DIR:PATH=/path/to/Slicer-SuperBuild-Release/Slicer-build/"

In that case, because the value of GIT_EXECUTABLE is set in
SlicerConfig (see r25523) and then mark_as_advanced is used
in FindGit module, it triggered https://gitlab.kitware.com/cmake/cmake/issues/15448

This commit workaround the problem commenting "mark_as_advanced"

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25552 3bd1e089-480b-0410-8dfb-8563597acbee