mergetools: avoid losing the merged version with meld

Add -o $output.
When using Meld as intended (merge from left and right into the center panel),
the merged version is written to the wrong file without this option ($base,
a temporary file, which is ignored by Mercurial).

Add meld.check=changed as a secondary safety net.

contrib/mergetools.hgrc

@@ -25,7 +25,8 b' merge.priority=-100'
 gpyfm.gui=True
 
 meld.gui=True
-meld.args=--label='local' $local --label='base' $base --label='other' $other
+meld.args=--label='local' $local --label='base' $base --label='other' $other -o $output
+meld.check=changed
 meld.diffargs=-a --label='$plabel1' $parent --label='$clabel' $child
 
 tkdiff.args=$local $other -a $base -o $output