Don't spuriously set mark on xref push #865
                
     Merged
            
            
          
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This fixes #863, whereby jumping to the definition of a symbol in the
same file was incorrectly activating the mark and region. This commit
also simplifies the original code, since the initial position marker can
simply be passed to xref-push-marker-stack. (The haskell-compat shim for
that function is also fixed here accordingly.)