filemerge: remove the hgmerge script
This is now better handled using [merge-tools] and [merge-patterns] in hgrc.
% should fail
abort: path contains illegal component: .hg/00changelog.i
adding a/a
% should fail
abort: path 'b/b' traverses symbolic link 'b'
% should succeed
% should still fail - maybe
abort: path 'b/b' traverses symbolic link 'b'