This bypasses the usual haskell file locking used to prevent opening a
file for read that is being written to.
This is unfortunately a bit of a hack. But it seems fairly unlikely to
get broken by changes to ghc. I hope. Using fdToHandle' will also work.
This does not work on windows because it uses openFd from posix. It
would probably be possible to implement it for windows too, just opening
the FD using the Win32 library instead. However, whether windows will
allow reading from a file that is also being written to I don't know,
and since in the git-annex case the writer could be another process (eg
external special remote), that might be doing its own locking in
windows, that seems a can of worms I'd prefer not to open.
