merge-one-file: remove stale comment

merge-one-file: remove stale comment

The "funny filename" comment was from b539c5e8fbd3 (git-merge-one:
new merge world order., 2005-12-07) where the removed code just
before that new comment ended with:

        merge "$4" "$orig" "$src2"

(yes, we used to use "merge" program from the RCS suite).  The
comment refers to one of the bad side effect the old code used to
have and warns against such a practice, i.e. it was talking about
the code that no longer existed.

Signed-off-by: Junio C Hamano <redacted>