GIT_DIR isn't unset in repo.method_missing

[bronson]

 def method_missing(name, *args, &block)
    cmd = name.to_s
    if cmd[0..3] == 'git_'
      ENV['GIT_DIR'] = path
      ... etc

The problem is, GIT_DIR remains set even after method_missing returns! This will cause trouble for any code that later shells out to git. For instance, my Ruby script might shell out to the Repo python script, then fail when Repo fork and execs a git process.

[minad]

fixed by 2e882f4