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Closing fds twice when using remote helpers


I started getting a weird error message during some test case involving
git-cinnabar, which is a remote-helper to access mercurial

The error says:
fatal: mmap failed: Bad file descriptor

... which was not making much sense. Some debugging later, and it turns
out this is what happens:

- start_command is called for fast-import
- start_command is called again for git-remote-hg, passing the
  fast_import->out as cmd->in.
- in start_command, we end up on the line of code that does
  close(cmd->in), so fast_import->out/cmd->in is now closed
- much later, in disconnect_helper, we call close(data->helper->out),
  where data->helper is the cmd for fast-import, and that fd was already
closed above.
- Except, well, fds being what they are, we in fact just closed a fd
  from a packed_git->pack_fd. So, when use_pack is later called, and
  tries to mmap data from that pack, it fails because the file
  descriptor was closed.

I'm not entirely sure how to address this... Any ideas?

Relatedly, use_pack calls xmmap, which does its own error handling and
die()s in case of error, but then goes on to do its own check with a
different error message (which, in fact, could be more useful in other
cases). It seems like it should call xmmap_gently instead.