Fix branch selection jumping back on background fetch (#4597)

- **PR Description**

When refreshing the branches list, we have code to keep the same branch
selected even when the refresh changes the sort order; this code
remembers the selected branch before the refresh, and then tries to
select it again afterwards (looking it up by name) if it is still there.

However, we stored the previously selected branch too early, before even
obtaining the branches list; if the user moved the selection between
that point and the end of the refresh, it would jump back. Fix this by
remembering the previous selection only at the last moment, right before
assigning the new branches slice.

We still have a race condition here between the UI code that manages the
selection as the user presses arrow keys, and the background thread
doing the refresh that reads and restores the selection; however, the
race was there before, and we make it neither better nor worse with this
PR. It doesn't seem to be a problem in practice.

Fixes #4116.

pkg/gui/controllers/helpers/refresh_helper.go

@@ -448,8 +448,6 @@ func (self *RefreshHelper) refreshBranches(refreshWorktrees bool, keepBranchSele
 	self.c.Mutexes().RefreshingBranchesMutex.Lock()
 	defer self.c.Mutexes().RefreshingBranchesMutex.Unlock()
 
-	prevSelectedBranch := self.c.Contexts().Branches.GetSelected()
-
 	reflogCommits := self.c.Model().FilteredReflogCommits
 	if self.c.Modes().Filtering.Active() && self.c.AppState.LocalBranchSortOrder == "recency" {
 		// in filter mode we filter our reflog commits to just those containing the path
@@ -484,6 +482,8 @@ func (self *RefreshHelper) refreshBranches(refreshWorktrees bool, keepBranchSele
 		self.c.Log.Error(err)
 	}
 
+	prevSelectedBranch := self.c.Contexts().Branches.GetSelected()
+
 	self.c.Model().Branches = branches
 
 	if refreshWorktrees {