Cache GitHub pull requests to improve performance

[geofflamrock]

Adds caching of GitHub pull requests to improve performance during updating a stack.

Details

The logic to update a stack got moved around in #328. A result of this is that we now get the status of a stack, including PRs multiple times, when syncing a stack (not updating) - once when we show the output and once to perform the update itself. This is to make the interface simpler for updating a stack.

I'm planning to change the way the underlying update works soon to not need to get a full status as we don't need the ahead/behind between branches to perform the update, we just need details like remote tracking branch and whether it exists, as well as the GitHub PR for cases when the remote branch doesn't get deleted. We'll probably still load the data multiple times but won't use the full status.

To avoid multiple requests to GitHub for pull requests, have added a caching implementation that decorates the inner one.

Background

• 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