Add ITK_GIT_TAG and ITK_GIT_REPOSITORY variables for Superbuild
Add ITK_GIT_TAG and ITK_GIT_REPOSITORY variables for Superbuild

Adding these advanced options to the SimpleITK Superbuild allows for
easy experimentation against development versions of ITK and usage of
faster repository. The ITK_REPOSITORY variable is now a legacy option
in favor of ITK_GIT_REPOSITORY.

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