Allow discarding changes only from local commits

We use CommitFilesController also for the files of commits that we show
elsewhere, e.g. for branch commits, tags, or stashes. It doesn't make sense to
discard changes from those (for stashes it might be possible to implement it
somehow, but that would be a new feature), so we disallow it unless we are in
the local commits panel.

pkg/integration/tests/stash/prevent_discarding_file_changes.go

@@ -0,0 +1,41 @@
+package stash
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var PreventDiscardingFileChanges = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Check that it is not allowed to discard changes to a file of a stash",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig:  func(config *config.AppConfig) {},
+	SetupRepo: func(shell *Shell) {
+		shell.EmptyCommit("initial commit")
+		shell.CreateFile("file", "content")
+		shell.GitAddAll()
+		shell.Stash("stash one")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Files().IsEmpty()
+
+		t.Views().Stash().
+			Focus().
+			Lines(
+				Contains("stash one").IsSelected(),
+			).
+			PressEnter()
+
+		t.Views().CommitFiles().
+			IsFocused().
+			Lines(
+				Contains("file").IsSelected(),
+			).
+			Press(keys.Universal.Remove)
+
+		t.ExpectPopup().Confirmation().
+			Title(Equals("Error")).
+			Content(Contains("Changes can only be discarded from local commits")).
+			Confirm()
+	},
+})