ExternalProject: Only run 'git fetch' when required.

In the current default update step for Git under the ExternalProject_Add
command, a 'git fetch' is always performed, followed by a 'git checkout' and
'git submodule update --recursive'.  However, a 'git fetch' can be time
consuming and requires a network connection.

To save time, we look at the current checked out hash, and only perform the
fetch if required.  This is performed in a CMake script so we can handle
the conditional logic in a cross platform manner.

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