work around buggy S_ISxxx(m) implementations

work around buggy S_ISxxx(m) implementations

There are buggy implementations of S_ISxxx(m) macros on some platforms
(e.g. NetBSD).  The issue is that NetBSD doesn't take care to wrap its
macro arguments in parentheses, so on Linux and sane systems we have
S_ISREG(m) defined as something like:

    (((m) & S_IFMT) == S_IFREG)

But on NetBSD:

    ((m & _S_IFMT) == _S_IFREG)

Since a caller in builtin/diff.c called our macro as `S_IFREG | 0644'
this bug introduced a logic error on NetBSD, since the precedence of
bit-wise & is higher than | in C.

[jc: took change description from Ævar Arnfjörð Bjarmason's patch]

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>