Fix branch selection jumping back on background fetch

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.

Author: stefanhaller

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 {