diff.*Prefix: use camelCase in the doc and test titles

diff.*Prefix: use camelCase in the doc and test titles

We added documentation for diff.srcPrefix and diff.dstPrefix with
their names properly camelCased, but the diff.noPrefix is listed
there in all lowercase.  Also these configuration variables, both
existing ones and the {src,dst}Prefix we recently added, were
spelled in all lowercase in the tests in t4013.

Now we are done with the main change, clean these up.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>