ll-merge: fix typo in comment

ll-merge: fix typo in comment

When a944af1d (merge: teach -Xours/-Xtheirs to binary ll-merge
driver, 2012-09-08) introduced FAVOR_OURS/FAVOR_THEIRS to the binary
ll-merge driver, it changed what happens to the merge result for the
outer merge, and updated the comment from:

    The tentative merge result is "ours" for the final round, or
    common ancestor for an internal merge.  Still return "conflicted
    merge" status.

to

    The tentative merge result is the or common ancestor for an
    internal merge.

What happened is obvious.  I noticed the lack of definitive article
in front of "common" but failed to remove "or".  Also I forgot to
describe what I did for the final merge, probably because I was
satisified by the description in the log message.

Signed-off-by: Junio C Hamano <gitster@pobox.com>