lstat_cache(): swap func(length, string) into func(string, length)

lstat_cache(): swap func(length, string) into func(string, length)

Swap function argument pair (length, string) into (string, length) to
conform with the commonly used order inside the GIT source code.

Also, add a note about this fact into the coding guidelines.

Signed-off-by: Kjetil Barvik <barvik@broadpark.no>
Signed-off-by: Junio C Hamano <gitster@pobox.com>