Server: Fork before clone. Fixes #6.

We `cd` into the cloned directory, which is fine for the command line utility,
but not for the server (its process should stay in the same directory).

Without this, we get, we get
  fatal: Could not get current working directory: No such file or directory
because we delete the temporary directories and so the server lost its cwd.
Merge pull request #10 from google/fork-before-cd

Server: Fork before clone. Fixes #6.
LGTM
Haddock fixes
Switch to using env variable for passing credentials. Fixes #8.

This is so that the credentials are not visible to other users
on the same system (e.g. show up in ps).