mergetools.hgrc: set vimdiff to check=changed

The standard reaction in from of unexpected vimdiff is to ":quit". This will
make vimdiff return a 0 status even if no merge were done at all.

This change detect that nothing have been changed in vimdiff as a potential
unresolved conflict.

contrib/mergetools.hgrc

@@ -16,6 +16,7 b' gvimdiff.regname=path'
 gvimdiff.priority=-9
 
 vimdiff.args=$local $other $base
+vimdiff.check=changed
 vimdiff.priority=-10
 
 merge.checkconflicts=True