Allow to switch branches in Commit View

When the user checks out a commit which has a local branch ref attached
to it, they can select between checking out the branch or checking out
the commit as detached head.

pkg/integration/tests/commit/checkout.go

@@ -0,0 +1,69 @@
+package commit
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var Checkout = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Checkout a commit as a detached head, or checkout an existing branch at a commit",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig:  func(config *config.AppConfig) {},
+	SetupRepo: func(shell *Shell) {
+		shell.EmptyCommit("one")
+		shell.EmptyCommit("two")
+		shell.NewBranch("branch1")
+		shell.NewBranch("branch2")
+		shell.EmptyCommit("three")
+		shell.EmptyCommit("four")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Commits().
+			Focus().
+			Lines(
+				Contains("four").IsSelected(),
+				Contains("three"),
+				Contains("two"),
+				Contains("one"),
+			).
+			PressPrimaryAction()
+
+		t.ExpectPopup().Menu().
+			Title(Contains("Checkout branch or commit")).
+			Lines(
+				Contains("Checkout branch").IsSelected(),
+				MatchesRegexp("Checkout commit [a-f0-9]+ as detached head"),
+				Contains("Cancel"),
+			).
+			Tooltip(Contains("Disabled: No branches found at selected commit.")).
+			Select(MatchesRegexp("Checkout commit [a-f0-9]+ as detached head")).
+			Confirm()
+		t.Views().Branches().Lines(
+			Contains("* (HEAD detached at"),
+			Contains("branch2"),
+			Contains("branch1"),
+			Contains("master"),
+		)
+
+		t.Views().Commits().
+			NavigateToLine(Contains("two")).
+			PressPrimaryAction()
+
+		t.ExpectPopup().Menu().
+			Title(Contains("Checkout branch or commit")).
+			Lines(
+				Contains("Checkout branch 'branch1'").IsSelected(),
+				Contains("Checkout branch 'master'"),
+				MatchesRegexp("Checkout commit [a-f0-9]+ as detached head"),
+				Contains("Cancel"),
+			).
+			Select(Contains("Checkout branch 'master'")).
+			Confirm()
+		t.Views().Branches().Lines(
+			Contains("master"),
+			Contains("branch2"),
+			Contains("branch1"),
+		)
+	},
+})