merge-tree: fix typo in "both changed identically"

Commit aacecc3 (merge-tree: don't print entries that match "local" -
2013-04-07) had a typo causing the "same in both" check to be incorrect
and check if both the base and "their" versions are removed instead of
checking that both the "our" and "their" versions are removed.  Fix
this.

Reported-by: René Scharfe <rene.scharfe@lsrfire.ath.cx>
Test-written-by: René Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>