do no-op post_commit test in wrapper

do no-op post_commit test in wrapper

This speeds up web commits by 1/4th of a second or so, since perl does
not have to start up for the post commit hook.

perl's locking is completly FuBar, since it's impossible to tell what perl
flock() really does, and thus difficult to write code in other languages
that interoperates with perl's locking. (Let alone interoperating with
existing fcntl locking from perl...)

In this particular case, I think I was able to find a way to avoid the
insanity, mostly. The C code does a true flock(2), and if perl is using an
incompatable lock method that does not use the same locking primative at
the kernel level, then the C code's test will fail, and it will go ahead
and run the perl code. Then the perl code's test will test the right thing.

On Debian, at least lately, perl's flock() does a true flock(2), so the
optimisation does work.