mergetools.hgrc: add minimal configuration for editmerge

The ``editmerge`` script is shipped in contrib and opens an editor on
every conflicting file. It needs minimal configuration to inject the
config marker in the file before opening. Otherwise it behaves the
same as ``internal:local`` and bad things happen.

contrib/mergetools.hgrc

@@ -71,6 +71,12 b' ecmerge.regkeyalt=Software\\Wow6432Node\\E'
 ecmerge.gui=True
 ecmerge.diffargs=$parent $child --mode=diff2 --title1='$plabel1' --title2='$clabel'
 
+# editmerge is a small script shipped in contrib.
+# It needs this config otherwise it behaves the same as internal:local
+editmerge.args=$output
+editmerge.check=changed
+editmerge.premerge=keep
+
 filemerge.executable=/Developer/Applications/Utilities/FileMerge.app/Contents/MacOS/FileMerge
 filemerge.args=-left $other -right $local -ancestor $base -merge $output
 filemerge.gui=True