Give better visual feedback when checking out the previous branch (#4929)

I find the command "Checkout previous branch" quite useful, and I use it
a lot. However, I'm unhappy with the visual feedback, so this PR
improves that a bit.

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.

Author: stefanhaller

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