improve temp dir security

http://bugs.debian.org/807341

* Fix insecure temporary permissions.

  Repair clones the git repository to a temp directory which is made
  using the user's umask. Thus, it might expose a git repo that is
  otherwise locked down.

* Fix potential denial of service attack when creating temp dirs.

  Since withTmpDir used easily predictable temporary directory names,
  an attacker could create foo.0, foo.1, etc and as long as it managed to
  keep ahead of it, could prevent it from ever returning.

  I'd rate this as a low utility DOS attack. Most attackers in a position
  to do this could just fill up the disk /tmp is on to prevent anything
  from writing temp files. And few parts of git-annex use withTmpDir
  anyway, so DOS potential is quite low.

Examined all callers of withTmpDir and satisfied myself that
switching to mkdtmp and so getting a mode 700 temp dir wouldn't break any
of them.

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