Add test for creating a fixup commit and squashing fixups

We have such a test already (squash_fixups_above_first_commit.go), but it can't
be used for what we want to check here, because it uses the first commit, and we
can't move down from there. So create a new one that basically does the same
thing, but for a commit in the middle. The focus of this new test is to check
how the selection behaves; as you can see, there is a problem both when creating
a fixup and when squashing fixups. We'll address these separately in the next
commits.

pkg/integration/tests/interactive_rebase/squash_fixups_above.go

@@ -0,0 +1,58 @@
+package interactive_rebase
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var SquashFixupsAbove = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Squashes all fixups above a commit and checks that the selected line stays correct.",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig:  func(config *config.AppConfig) {},
+	SetupRepo: func(shell *Shell) {
+		shell.
+			CreateNCommits(3).
+			CreateFileAndAdd("fixup-file", "fixup content")
+	},
+	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.Commits.CreateFixupCommit).
+			Tap(func() {
+				t.ExpectPopup().Confirmation().
+					Title(Equals("Create fixup commit")).
+					Content(Contains("Are you sure you want to create a fixup! commit for commit")).
+					Confirm()
+			}).
+			Lines(
+				Contains("fixup! commit 02"),
+				Contains("commit 03").IsSelected(), // wrong, we want the next line
+				Contains("commit 02"),
+				Contains("commit 01"),
+			).
+			SelectNextItem().
+			Press(keys.Commits.SquashAboveCommits).
+			Tap(func() {
+				t.ExpectPopup().Menu().
+					Title(Equals("Apply fixup commits")).
+					Select(Contains("Above the selected commit")).
+					Confirm()
+			}).
+			Lines(
+				Contains("commit 03"),
+				Contains("commit 02"),
+				Contains("commit 01").IsSelected(), // wrong, we want the previous line
+			).
+			SelectPreviousItem()
+
+		t.Views().Main().
+			Content(Contains("fixup content"))
+	},
+})

pkg/integration/tests/test_list.go

@@ -188,6 +188,7 @@ var tests = []*components.IntegrationTest{
 	interactive_rebase.RewordYouAreHereCommitWithEditor,
 	interactive_rebase.SquashDownFirstCommit,
 	interactive_rebase.SquashDownSecondCommit,
+	interactive_rebase.SquashFixupsAbove,
 	interactive_rebase.SquashFixupsAboveFirstCommit,
 	interactive_rebase.SquashFixupsInCurrentBranch,
 	interactive_rebase.SwapInRebaseWithConflict,