optimization: avoid a fetch when checkout out a pull request

In #4057 @niik mentions an opportunity to avoid a fetch if we know the branch exists:

      // TODO: I think we could skip this fetch if we know that we have the branch locally
      // already. That way we'd match the behavior of checking out a branch.
      if (defaultRemote) {
        await this._fetchRemote(
          repository,
          defaultRemote,
          FetchType.UserInitiatedTask
        )
      }

I’ll let @niik add any additional thoughts on the best approach here.

[iAmWillShepherd]

@agisilaos. Go for it! I’ll add it to our project board for tracking issues our community have claimed.

[niik]

Before I start working on this, I would love to know your additional thoughts or any other info that may help me during the process.

I believe I was referring to using the same logic that already exists in the forked-case: https://github.com/desktop/desktop/blob/321c3de0f143e29cfaab702dbc6e5766f8d91fda/app/src/lib/stores/app-store.ts#L3323-L3333

So I’d say the work consists of extracting that logic into something reusable and then verifying that my assumption was correct in the first place.