Add tests for applying a patch when there's a modified file

The tests show that this currently fails with the confusing error message "does
not match index", regardless of whether the patch conflicts with the
modifications or not. We'll improve this in the next commit.

I don't bother adding tests for reverting a patch, as the code is basically the
same as for apply.

pkg/integration/tests/patch_building/apply_with_modified_file_conflict.go

@@ -0,0 +1,60 @@
+package patch_building
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var ApplyWithModifiedFileConflict = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Apply a custom patch, with a modified file in the working tree that conflicts with the patch",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig:  func(config *config.AppConfig) {},
+	SetupRepo: func(shell *Shell) {
+		shell.NewBranch("branch-a")
+		shell.CreateFileAndAdd("file1", "1\n2\n3\n")
+		shell.Commit("first commit")
+
+		shell.NewBranch("branch-b")
+		shell.UpdateFileAndAdd("file1", "11\n2\n3\n")
+		shell.Commit("update")
+
+		shell.Checkout("branch-a")
+		shell.UpdateFile("file1", "111\n2\n3\n")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Branches().
+			Focus().
+			Lines(
+				Contains("branch-a").IsSelected(),
+				Contains("branch-b"),
+			).
+			Press(keys.Universal.NextItem).
+			PressEnter()
+
+		t.Views().SubCommits().
+			IsFocused().
+			Lines(
+				Contains("update").IsSelected(),
+				Contains("first commit"),
+			).
+			PressEnter()
+
+		t.Views().CommitFiles().
+			IsFocused().
+			Lines(
+				Equals("M file1").IsSelected(),
+			).
+			PressPrimaryAction()
+
+		t.Views().Information().Content(Contains("Building patch"))
+
+		t.Views().Secondary().Content(Contains("-1\n+11\n"))
+
+		t.Common().SelectPatchOption(MatchesRegexp(`Apply patch$`))
+
+		t.ExpectPopup().Alert().Title(Equals("Error")).
+			Content(Equals("error: file1: does not match index")).
+			Confirm()
+	},
+})

pkg/integration/tests/patch_building/apply_with_modified_file_no_conflict.go

@@ -0,0 +1,60 @@
+package patch_building
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var ApplyWithModifiedFileNoConflict = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Apply a custom patch, with a modified file in the working tree that does not conflict with the patch",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig:  func(config *config.AppConfig) {},
+	SetupRepo: func(shell *Shell) {
+		shell.NewBranch("branch-a")
+		shell.CreateFileAndAdd("file1", "1\n2\n3\n")
+		shell.Commit("first commit")
+
+		shell.NewBranch("branch-b")
+		shell.UpdateFileAndAdd("file1", "1\n2\n3\n4\n")
+		shell.Commit("update")
+
+		shell.Checkout("branch-a")
+		shell.UpdateFile("file1", "11\n2\n3\n")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Branches().
+			Focus().
+			Lines(
+				Contains("branch-a").IsSelected(),
+				Contains("branch-b"),
+			).
+			Press(keys.Universal.NextItem).
+			PressEnter()
+
+		t.Views().SubCommits().
+			IsFocused().
+			Lines(
+				Contains("update").IsSelected(),
+				Contains("first commit"),
+			).
+			PressEnter()
+
+		t.Views().CommitFiles().
+			IsFocused().
+			Lines(
+				Equals("M file1").IsSelected(),
+			).
+			PressPrimaryAction()
+
+		t.Views().Information().Content(Contains("Building patch"))
+
+		t.Views().Secondary().Content(Contains("3\n+4"))
+
+		t.Common().SelectPatchOption(MatchesRegexp(`Apply patch$`))
+
+		t.ExpectPopup().Alert().Title(Equals("Error")).
+			Content(Equals("error: file1: does not match index")).
+			Confirm()
+	},
+})

pkg/integration/tests/test_list.go

@@ -300,6 +300,8 @@ var tests = []*components.IntegrationTest{
 	patch_building.Apply,
 	patch_building.ApplyInReverse,
 	patch_building.ApplyInReverseWithConflict,
+	patch_building.ApplyWithModifiedFileConflict,
+	patch_building.ApplyWithModifiedFileNoConflict,
 	patch_building.EditLineInPatchBuildingPanel,
 	patch_building.MoveRangeToIndex,
 	patch_building.MoveToEarlierCommit,