cache repository refresh to reduce load when refreshing takes time

[shiftkey]

As part of investigating performance improvements, I noticed two cases where users may find the app running multiple refreshes. Here's a log to illustrate what this looks like with tracing enabled:

Two particular cases stood out around the user interacting with the app:

• the initial launch of the app - 2 or 3 refreshes can be initiated, depending on the setup
• switching back and forth between Desktop and another app - app focus will always trigger a refresh, irrespective of the current state of the repository (which might already be refreshing)

On systems with slow I/O or complex repositories (also Windows will see this more acutely), the overall refresh operation may take multiple seconds. Adding more in-flight refreshes will only slow down the first refresh, leading to a sort of "self denial of service" in the app if the user is able to trigger these continually by interacting with the app.

To reduce the impact of Desktop's refreshing work we could track the current refresh operation in a per-repository cache, and if a refresh is active then a new refresh should not be started.

For reference we did something similar in #7501 for the pull request refresh:

desktop/app/src/lib/stores/pull-request-store.ts

Lines 71 to 94 in bee4c6b

	public refreshPullRequests(repo: GitHubRepository, account: Account) {
	const dbId = forceUnwrap("Can't refresh PRs, no dbId", repo.dbID)
	const currentOp = this.currentRefreshOperations.get(dbId)
	if (currentOp !== undefined) {
	return currentOp
	}
	this.lastRefreshForRepository.set(dbId, Date.now())
	this.emitIsLoadingPullRequests(repo, true)
	const promise = this.fetchAndStorePullRequests(repo, account)
	.catch(err => {
	log.error(`Error refreshing pull requests for '${repo.fullName}'`, err)
	})
	.then(() => {
	this.currentRefreshOperations.delete(dbId)
	this.emitIsLoadingPullRequests(repo, false)
	})
	this.currentRefreshOperations.set(dbId, promise)
	return promise
	}