Utilities/Release: Fetch stage/master/head

Update our release scripts to fetch `refs/stage/master/head` from
the `gitlab.kitware.com` repository.  This will allow us to prepare
releases from staged commits before merging them to `master` or
`release`.

Utilities/Release/release_cmake.cmake

@@ -40,7 +40,8 @@ elseif(CMAKE_CREATE_VERSION STREQUAL "nightly")
   set(GIT_FETCH "${GIT_COMMAND} fetch origin refs/${nightly}:refs/remotes/origin/${nightly}")
   set(GIT_BRANCH origin/${nightly})
 else()
-  set(GIT_FETCH "")
+  set(stage stage/master/head)
+  set(GIT_FETCH "${GIT_COMMAND} fetch origin refs/${stage}:refs/remotes/origin/${stage}")
   set(GIT_BRANCH ${CMAKE_CREATE_VERSION})
 endif()