scripts: equality test '==' is not portable.

scripts: equality test '==' is not portable.

On NetBSD 3 we trigger an error:

    [: ==: unexpected operator

Double-equal is accepted by bash built-in '[' and bash(1) suggests
using '=' for strict POSIX compliance (test(1) from coreutils does not
mention '==').  Eradicate their uses everywhere.

[jc: Somebody with a pseudonym kindly sent a message to let
 me know about the problem privately; I do not have access to a NetBSD
 box.]

Signed-off-by: Junio C Hamano <junkio@cox.net>