merge-ort: lowercase a few error messages

merge-ort: lowercase a few error messages

As noted in CodingGuidelines, error messages should not be capitalized.
Fix up a few of these that were copied verbatim from merge-recursive to
match our modern style.

We'll likewise fix up the matching ones from merge-recursive. We care a
bit less there, since the hope is that it will eventually go away. But
besides being the right thing to do in the meantime, it is necessary for
t6406 to pass both with and without GIT_TEST_MERGE_ALGORITHM set (one of
our CI jobs sets it to "recursive", which will use the merge-recursive.c
code). An alternative would be to use "grep -i" in the test to check
the message, but it's nice for the test suite to be be more exact (we'd
notice if the capitalization fix regressed).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>