Move PullChanges to separate interface to improve testing

[geofflamrock]

Introduces IStackActions and moves StackHelpers.PullChanges to improve the ability to test commands without needing to have a full Git repo.

Part of a series of PRs:

• Cleanup unused methods in GitClient #323
• Move PullChanges to separate interface to improve testing #326
  • Move PushChanges to separate interface to improve testing #327
    • Move UpdateStack to separate interface to improve testing #328
      • Cache GitHub pull requests to improve performance #329
• Refactor delete stack tests to not need a full Git repo #330
• Refactor new stack tests to not use full Git repo #332
• Refactor stack switch tests to not use full Git repo #333
• Refactor add branch tests to not use full Git repo #341
• Refactor new branch tests to not use full Git repo #342
• Refactor stack status tests to not use a full Git repo #346
• Change more push stack tests to not use a full Git repo #347
• Change create pull requests tests to not use full Git repo #349