Give better feedback when checking out the previous branch

Previously, the feedback you got when pressing "-" was just a "Checking out..."
status in the bottom line. This was both easy to miss if you are used to looking
for an inline status in the branches panel, and it didn't provide information
about which branch was being checked out, which can be annoying in very large
repos where checking out takes a while, and you only see at the end if you are
now on the right branch.

Improve this by trying to figure out which branch was the previously checked out
one, and then checking it out normally so that you get an inline status next to
it (as if you had pressed space on it). There are cases where this fails, e.g.
when the previously checked out ref was a detached head, in which case we fall
back to the previous behavior.

pkg/commands/git_commands/branch.go

@@ -116,6 +116,22 @@ func (self *BranchCommands) CurrentBranchName() (string, error) {
 	return strings.TrimSpace(output), nil
 }
 
+// Gets the full ref name of the previously checked out branch. Can return an empty string (but no
+// error) e.g. when the previously checked out thing was a detached head.
+func (self *BranchCommands) PreviousRef() (string, error) {
+	cmdArgs := NewGitCmd("rev-parse").
+		Arg("--symbolic-full-name").
+		Arg("@{-1}").
+		ToArgv()
+
+	output, err := self.cmd.New(cmdArgs).DontLog().RunWithOutput()
+	if err != nil {
+		return "", err
+	}
+
+	return strings.TrimSpace(output), nil
+}
+
 // LocalDelete delete branch locally
 func (self *BranchCommands) LocalDelete(branches []string, force bool) error {
 	cmdArgs := NewGitCmd("branch").

pkg/gui/controllers/branches_controller.go

@@ -490,7 +490,7 @@ func (self *BranchesController) forceCheckout() error {
 
 func (self *BranchesController) checkoutPreviousBranch() error {
 	self.c.LogAction(self.c.Tr.Actions.CheckoutBranch)
-	return self.c.Helpers().Refs.CheckoutRef("-", types.CheckoutRefOptions{})
+	return self.c.Helpers().Refs.CheckoutPreviousRef()
 }
 
 func (self *BranchesController) checkoutByName() error {

pkg/gui/controllers/helpers/refs_helper.go

@@ -163,6 +163,15 @@ func (self *RefsHelper) CheckoutRemoteBranch(fullBranchName string, localBranchN
 	})
 }
 
+func (self *RefsHelper) CheckoutPreviousRef() error {
+	previousRef, err := self.c.Git().Branch.PreviousRef()
+	if err == nil && strings.HasPrefix(previousRef, "refs/heads/") {
+		return self.CheckoutRef(strings.TrimPrefix(previousRef, "refs/heads/"), types.CheckoutRefOptions{})
+	}
+
+	return self.CheckoutRef("-", types.CheckoutRefOptions{})
+}
+
 func (self *RefsHelper) GetCheckedOutRef() *models.Branch {
 	if len(self.c.Model().Branches) == 0 {
 		return nil