Fix quoting of args to subprocess.check_command and in debug prints.

Signed-off-by: Daira Hopwood <daira@leastauthority.com>
Use os.path.isabs rather than .startswith('/').

Signed-off-by: Daira Hopwood <daira@leastauthority.com>
Explicitly use the right private key for git.

Signed-off-by: Daira Hopwood <daira@leastauthority.com>