Ensure we only check one repo's stash size at a time from refreshRepository

[outofambit]

(from investigation by @niik)

While repositories are refreshed serially the actual metric update check is not awaited meaning that theoretically more than one stash check could run simultaneously.

---

(from me, @outofambit)

This

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

Line 2116 in 70f9bea

this.updateStashEntryCountMetric(repository)

is where the metric update check is run (and not await-ed).

The tradeoff here is that if we await that call, the results of _refreshRepository, we block other sequential operations that come afterwards until that check is finished. (_refreshRepository is await-ed in a few key places, like checkout, commit, and pull). However, since we limit ourselves to one metric update check per 24 hours, this will probably not result in a noticeable performance difference.