git-resource: HTTPS auth doesn't work

Git resources using https auth don’t seem to function at all.

Using this setup:

resources:
- name: reponame
  type: git
  source:
    uri: https://github.com/companyname/reponame.git
    branch: master
    username: githubusername
    password: githubpassword

The check script fails with the following error message:

resource script '/opt/resource/check []' failed: exit status 128

stderr:
Cloning into '/tmp/git-resource-repo-cache'...
fatal: could not read Username for 'https://github.com': No such device or address

HTTPS auth is important because you can’t authenticate submodules using private keys. Thanks!