merge-tree: don't print entries that match "local"

The documentation says:

	the output from the command omits entries that match the
	<branch1> tree.

But currently "added in branch1" and "removed in branch1" (both while
unchanged in branch2) do print output.  Change this so that the
behaviour matches the documentation.

Signed-off-by: John Keeping <john@keeping.me.uk>