bump rename limit defaults

bump rename limit defaults

The current rename limit default of 100 was arbitrarily
chosen. Testing[1] has shown that on modern hardware, a
limit of 200 adds about a second of computation time, and a
limit of 500 adds about 5 seconds of computation time.

This patch bumps the default limit to 200 for viewing diffs,
and to 500 for performing a merge. The limit for generating
git-status templates is set independently; we bump it up to
200 here, as well, to match the diff limit.

[1]: See <20080211113516.GB6344@coredump.intra.peff.net>

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