Better error message for trying to squash or fixup the first commit

It's not so much the total number of commits that matters here, it's just
whether we are on the first one. (This includes the other condition.)

This allows us to get rid of the condition in rebase.go.

pkg/integration/tests/interactive_rebase/fixup_first_commit.go

@@ -0,0 +1,37 @@
+package interactive_rebase
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var FixupFirstCommit = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Tries to fixup the first commit, which results in an error message",
+	ExtraCmdArgs: "",
+	Skip:         false,
+	SetupConfig:  func(config *config.AppConfig) {},
+	SetupRepo: func(shell *Shell) {
+		shell.
+			CreateNCommits(2)
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Commits().
+			Focus().
+			Lines(
+				Contains("commit 02"),
+				Contains("commit 01"),
+			).
+			NavigateToListItem(Contains("commit 01")).
+			Press(keys.Commits.MarkCommitAsFixup).
+			Tap(func() {
+				t.ExpectPopup().Alert().
+					Title(Equals("Error")).
+					Content(Equals("There's no commit below to squash into")).
+					Confirm()
+			}).
+			Lines(
+				Contains("commit 02"),
+				Contains("commit 01"),
+			)
+	},
+})