diff: use "find" instead of "detect" as prefix for long forms of -M and -C

diff: use "find" instead of "detect" as prefix for long forms of -M and -C

It is more consistent with existing --find-copies-harder; luckily "detect"
variant has not appeared in any officially released version of git.

Signed-off-by: Yann Dirson <redacted>
Signed-off-by: Junio C Hamano <redacted>