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