Support scanning multiple git repositories in one invocation

[domanchi]

Description

With the changes made in #188, this supports scanning for multiple git repositories at once.

There are several benefits to this:

1. With this, you can generate one giant baseline for all repositories that you track. This is especially handy for plugin development, when you want to compare baselines across multiple repositories.

2. Scanning multiple repositories, without needing to scan every file (with --all-files flag).

I also threw in a bug fix to ignore files, if no git tracked files are found.

Example

~ $ git status
fatal: Not a git repository (or any parent up to mount point /home/aaronloo)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
~ $ detect-secrets scan detect-secrets/test_data detect-secrets-server
{
  "exclude": {
    "files": null,
    "lines": null
  },
  "generated_at": "2019-06-15T02:59:26Z",
  "plugins_used": [...],
  "results": {
    "detect-secrets-server/README.md": [
      {
        "hashed_secret": "cdd5f4b553c63f87de1a4fce14f1ad8fa2a1d1f6",
        "line_number": 191,
        "type": "Hex High Entropy String"
      }
    ],
    ...
    "detect-secrets/test_data/config.env": [
      {
        "hashed_secret": "513e0a36963ae1e8431c041b744679ee578b7c44",
        "line_number": 1,
        "type": "Base64 High Entropy String"
      }
    ]
}

[KevinHock]

LGTM ☕️ ✅🛳