Add test to auto-amend a commit after pressing e on it

Auto-amending is a little-known feature of git that is very convenient once you
know it: whenever you stop at a commit marked with `edit` in an interactive
rebase, you can make changes and stage them, and when you continue the rebase
they automatically get amended to the commit you had stopped at. This is so
convenient because making changes to a commit is one of the main reasons why you
edit a commit.

Unfortunately this currently doesn't work in lazygit because we don't actually
use `edit` to stop at the first commit (instead, we add a `break` todo after it,
which doesn't have the auto-amend functionality).

We'll improve this later in this branch.

pkg/integration/tests/interactive_rebase/edit_and_auto_amend.go

@@ -0,0 +1,58 @@
+package interactive_rebase
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var EditAndAutoAmend = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Edit a commit, make a change and stage it, then continue the rebase to auto-amend the commit",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig:  func(config *config.AppConfig) {},
+	SetupRepo: func(shell *Shell) {
+		shell.
+			CreateNCommits(3)
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Commits().
+			Focus().
+			Lines(
+				Contains("commit 03"),
+				Contains("commit 02"),
+				Contains("commit 01"),
+			).
+			NavigateToLine(Contains("commit 02")).
+			Press(keys.Universal.Edit).
+			Lines(
+				Contains("commit 03"),
+				MatchesRegexp("YOU ARE HERE.*commit 02").IsSelected(),
+				Contains("commit 01"),
+			)
+
+		t.Shell().CreateFile("fixup-file", "fixup content")
+		t.Views().Files().
+			Focus().
+			Press(keys.Files.RefreshFiles).
+			Lines(
+				Contains("??").Contains("fixup-file").IsSelected(),
+			).
+			PressPrimaryAction()
+
+		t.Common().ContinueRebase()
+
+		t.Views().Commits().
+			Focus().
+			Lines(
+				Contains("commit 03"),
+				Contains("commit 02").IsSelected(),
+				Contains("commit 01"),
+			)
+
+		t.Views().Main().
+			/* EXPECTED:
+			Content(Contains("fixup content"))
+			ACTUAL: */
+			Content(DoesNotContain("fixup content"))
+	},
+})

pkg/integration/tests/test_list.go

@@ -210,6 +210,7 @@ var tests = []*components.IntegrationTest{
 	interactive_rebase.DropCommitInCopiedBranchWithUpdateRef,
 	interactive_rebase.DropTodoCommitWithUpdateRef,
 	interactive_rebase.DropWithCustomCommentChar,
+	interactive_rebase.EditAndAutoAmend,
 	interactive_rebase.EditFirstCommit,
 	interactive_rebase.EditLastCommitOfStackedBranch,
 	interactive_rebase.EditNonTodoCommitDuringRebase,