variables available: `$LOCAL` is set to the name of the temporary
file containing the contents of the diff pre-image and `$REMOTE`
is set to the name of the temporary file containing the contents
-of the diff post-image. `$BASE` is provided for compatibility
-with custom merge tool commands and has the same value as `$LOCAL`.
+of the diff post-image. `$MERGED` is the name of the file which is
+being compared. `$BASE` is provided for compatibility
+with custom merge tool commands and has the same value as `$MERGED`.
-x <command>::
--extcmd=<command>::