blame: prefer xsnprintf to strcpy for colors

blame: prefer xsnprintf to strcpy for colors

Our color buffers are all COLOR_MAXLEN, which fits the
largest possible color. So we can never overflow the buffer
by copying an existing color. However, using strcpy() makes
it harder to audit the code-base for calls that _are_
problems. We should use something like xsnprintf(), which
shows the reader that we expect this never to fail (and
provides a run-time assertion if it does, just in case).

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