Add a test for staging a line with diff.mnemonicPrefix set

If diff.mnemonicPrefix is set, pressing space on a line in the focused
main view does nothing.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

pkg/integration/tests/main_view/stage_diff_lines_with_mnemonic_prefixes.go

@@ -0,0 +1,51 @@
+package main_view
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var StageDiffLinesWithMnemonicPrefixes = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Stage a line from the focused main view when diff.mnemonicPrefix is set",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig: func(cfg *config.AppConfig) {
+		cfg.GetUserConfig().Gui.UseHunkModeInDiffView = false
+	},
+	SetupRepo: func(shell *Shell) {
+		shell.CreateFileAndAdd("file1", "one\ntwo\nthree\n")
+		shell.Commit("one")
+
+		shell.UpdateFile("file1", "one\nADD1\nADD2\ntwo\nthree\n")
+
+		// git then puts i/ and w/ in front of the paths of the working tree's diff
+		// instead of a/ and b/.
+		shell.SetConfig("diff.mnemonicPrefix", "true")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Files().
+			IsFocused().
+			Lines(
+				Contains("file1").IsSelected(),
+			).
+			Press(keys.Universal.FocusMainView)
+
+		// The user chose these prefixes, so the diff shows them.
+		t.Views().Main().
+			IsFocused().
+			ContainsLines(
+				Equals("diff --git i/file1 w/file1"),
+			).
+			SelectedLines(
+				Contains("+ADD1"),
+			).
+			PressPrimaryAction()
+
+		t.Views().Files().Lines(
+			/* EXPECTED:
+			Contains("MM file1"),
+			ACTUAL: */
+			Contains(" M file1"),
+		)
+	},
+})

pkg/integration/tests/test_list.go

@@ -489,6 +489,7 @@ var tests = []*components.IntegrationTest{
 	main_view.StageDiffLines,
 	main_view.StageDiffLinesOfAPathWithASpace,
 	main_view.StageDiffLinesWithCustomPrefixes,
+	main_view.StageDiffLinesWithMnemonicPrefixes,
 	main_view.StageDiffLinesWithNoPrefixes,
 	main_view.StageHunksWithRapidKeypresses,
 	main_view.StagePartialBlockOfChangesFirstLines,