GitSetup: Tell Git not to export .gitattributes

GitSetup upstream added an attribute to export `.gitattributes` so that
importing snapshots into the sources of other projects would bring along
the attributes.  However, we don't want to export them from CMake.  Drop
`.gitattributes` entries not relevant to CMake.

Utilities/GitSetup/.gitattributes

 .git*            export-ignore
-.gitattributes  -export-ignore
 
 config*          eol=lf         whitespace=indent-with-non-tab
 git-*            eol=lf         whitespace=indent-with-non-tab