cvsserver: Implement update -p (print to stdout)

cvsserver: Implement update -p (print to stdout)

Cvs update -p -r <rev> <path> is the documented way to retrieve a
specific revision of a file (similar to git show <rev>:<path>).
Without this patch, the -p flag is ignored and status output is
produced, causing clients to interpret it as the contents of the file.

TkCVS uses update -p as a basis for implementing its various "View"
and "Diff" commands.

Signed-off-by: Damien Diederen <dash@foobox.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>