merge-recursive: fix copy-paste mistake

merge-recursive: fix copy-paste mistake

The following issue was found by scan.coverity.com (ID: 1049510),
and claimed to be likely a copy-paste mistake.

Introduced in 331a1838b (2010-07-02, Try normalizing files
to avoid delete/modify conflicts when merging), which is
quite a long time ago, so I'm rather unsure if it's of any impact
or just went unnoticed.

The line after the changed line has a comparison of 'o.len' to 'a.len',
so we should assume the lengths may be different.

I'd be happy to have a test for this bug(?) attached to
t6031-merge-recursive.sh, but I did not manage to
come up with a test in a reasonable amount of time.

Signed-off-by: Stefan Beller <stefanbeller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>