give worst-case 'merge' merge-tool lowest priority

Another tool had -10 already.  Since 'merge' is clearly a worst-case
tool (internal), lowering to -100 ensures there's plenty of room for
slightly better cases.

contrib/mergetools.hgrc

@@ -13,7 +13,7 b' gvimdiff.regname=path'
 gvimdiff.priority=-9
 
 merge.checkconflicts=True
-merge.priority=-10
+merge.priority=-100
 
 gpyfm.gui=True