Switch to branches view when checking out a commit

We move the code to push the branches context into CheckoutRef, this way it
works consistently no matter where we call it from. Previously, checking out
remote branches or tags would switch to the branches view, but checking out a
commit did not.

Note that it now also takes effect for undoing or redoing a checkout, which may
be a bit questionable; but I still think it makes sense for this, too.

pkg/integration/tests/commit/checkout.go

@@ -43,14 +43,16 @@ var Checkout = NewIntegrationTest(NewIntegrationTestArgs{
 			Select(MatchesRegexp("Checkout commit [a-f0-9]+ as detached head")).
 			Confirm()
 		t.Views().Branches().
+			IsFocused().
 			Lines(
-				Contains("* (HEAD detached at"),
+				Contains("* (HEAD detached at").IsSelected(),
 				Contains("branch1"),
 				Contains("branch2"),
 				Contains("master"),
 			)
 
 		t.Views().Commits().
+			Focus().
 			NavigateToLine(Contains("two")).
 			PressPrimaryAction()
 
@@ -65,8 +67,9 @@ var Checkout = NewIntegrationTest(NewIntegrationTestArgs{
 			Select(Contains("Checkout branch 'master'")).
 			Confirm()
 		t.Views().Branches().
+			IsFocused().
 			Lines(
-				Contains("master"),
+				Contains("master").IsSelected(),
 				Contains("branch1"),
 				Contains("branch2"),
 			)