git p4: scrub crlf for utf16 files on windows

git p4: scrub crlf for utf16 files on windows

Files of type utf16 are handled with "p4 print" instead of the
normal "p4 -G print" interface due to how the latter does not
produce correct output.  See 55aa571 (git-p4: handle utf16
filetype properly, 2011-09-17) for details.

On windows, though, "p4 print" can not be told which line
endings to use, as there is no underlying client, and always
chooses crlf, even for utf16 files.  Convert the \r\n into \n
when importing utf16 files.

The fix for this is complex, in that the problem is a property
of the NT version of p4.  There are old versions of p4 that
were compiled directly for cygwin that should not be subjected
to text replacement.  The right check here, then, is to look
at the p4 version, not the OS version.  Note also that on cygwin,
platform.system() is "CYGWIN_NT-5.1" or similar, not "Windows".

Add a function to memoize the p4 version string and use it to
check for "/NT", indicating the Windows build of p4.

Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>