Add test demonstrating selection bug when staging a range of lines

The selected line is not in the right position after staging a range of lines;
see next commit.

pkg/integration/tests/commit/stage_range_of_lines.go

@@ -0,0 +1,42 @@
+package commit
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var StageRangeOfLines = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Staging a range of lines",
+	ExtraCmdArgs: "",
+	Skip:         false,
+	SetupConfig:  func(config *config.AppConfig) {},
+	SetupRepo: func(shell *Shell) {
+		shell.CreateFileAndAdd("myfile", "1st\n2nd\n3rd\n4th\n5th\n6th\n")
+		shell.Commit("Add file")
+		shell.UpdateFile("myfile", "1st changed\n2nd changed\n3rd\n4th\n5th changed\n6th\n")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Files().
+			IsFocused().
+			PressEnter()
+
+		t.Views().Staging().
+			Content(
+				Contains("-1st\n-2nd\n+1st changed\n+2nd changed\n 3rd\n 4th\n-5th\n+5th changed\n 6th"),
+			).
+			SelectedLine(Equals("-1st")).
+			Press(keys.Main.ToggleDragSelect).
+			SelectNextItem().
+			SelectNextItem().
+			SelectNextItem().
+			SelectNextItem().
+			PressPrimaryAction().
+			Content(
+				Contains(" 3rd\n 4th\n-5th\n+5th changed\n 6th"),
+			).
+			/* EXPECTED:
+			SelectedLine(Equals("-5th"))
+			ACTUAL */
+			SelectedLine(Equals("+5th changed"))
+	},
+})

pkg/integration/tests/tests.go

@@ -52,6 +52,7 @@ var tests = []*components.IntegrationTest{
 	commit.Unstaged,
 	commit.StagedWithoutHooks,
 	commit.DiscardOldFileChange,
+	commit.StageRangeOfLines,
 	custom_commands.Basic,
 	custom_commands.FormPrompts,
 	custom_commands.MenuFromCommand,