Add regression test for staging a single added line

This is a pretty special case (see comment in the test for details). It is
working correctly, but I almost broke it during development, so it's good to
cover it with a test.

pkg/integration/tests/staging/select_next_line_after_staging_isolated_added_line.go

@@ -0,0 +1,51 @@
+package staging
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+// Tests that after staging an isolated addition (one that is alone in its block of changes), the
+// cursor stays at the first change of the next block of changes which moves up to the same line,
+// even if that block starts with a deletion.
+var SelectNextLineAfterStagingIsolatedAddedLine = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "After staging an isolated added line, the cursor advances to the next hunk's first change",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig: func(config *config.AppConfig) {
+		config.GetUserConfig().Gui.UseHunkModeInStagingView = false
+	},
+	SetupRepo: func(shell *Shell) {
+		shell.CreateFileAndAdd("file1", "1\n2\n3\n4\n5\n6\n7\n8\n9\n")
+		shell.Commit("one")
+
+		shell.UpdateFile("file1", "1\n2\n3\nnew\n4\n5\n6\n7b\n8\n9\n")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Files().
+			IsFocused().
+			Lines(
+				Contains("file1").IsSelected(),
+			).
+			PressEnter()
+
+		t.Views().Staging().
+			IsFocused().
+			ContainsLines(
+				Contains(" 1"),
+				Contains(" 2"),
+				Contains(" 3"),
+				Contains("+new"),
+				Contains(" 4"),
+				Contains(" 5"),
+				Contains(" 6"),
+				Contains("-7"),
+				Contains("+7b"),
+				Contains(" 8"),
+				Contains(" 9"),
+			).
+			SelectedLine(Contains("+new")).
+			PressPrimaryAction().
+			SelectedLine(Contains("-7"))
+	},
+})

pkg/integration/tests/test_list.go

@@ -377,6 +377,7 @@ var tests = []*components.IntegrationTest{
 	staging.DiffContextChange,
 	staging.DiscardAllChanges,
 	staging.Search,
+	staging.SelectNextLineAfterStagingIsolatedAddedLine,
 	staging.StageHunks,
 	staging.StageLines,
 	staging.StagePartialBlockOfChangesFirstLines,