Show an error if the commit to discard lines from can't be found

Discarding lines from a commit's diff rewrites the commit with a
rebase. For this we look up the commit among the commits of the
current branch. If it isn't there, pressing the key does nothing. The
disabled reason only allows discarding in a diff of the commits panel,
so this shouldn't happen. If it does, the user should see it.

Show an error that names the commit instead.

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

pkg/gui/controllers/commit_diff_actions.go

@@ -1,6 +1,7 @@
 package controllers
 
 import (
+	"errors"
 	"fmt"
 
 	"github.com/jesseduffield/generics/set"
@@ -9,6 +10,7 @@ import (
 	"github.com/jesseduffield/lazygit/pkg/commands/patch"
 	"github.com/jesseduffield/lazygit/pkg/gocui"
 	"github.com/jesseduffield/lazygit/pkg/gui/types"
+	"github.com/jesseduffield/lazygit/pkg/utils"
 	"github.com/samber/lo"
 )
 
@@ -236,7 +238,8 @@ func (self *CommitDiffActions) DiscardSelection(pane types.DiffPaneContext, firs
 	}
 	commitIndex := self.indexOfTargetCommit(target)
 	if commitIndex == -1 {
-		return nil
+		return errors.New(utils.ResolvePlaceholderString(self.c.Tr.CommitNotInCommitsPanel,
+			map[string]string{"hash": utils.ShortHash(target.to)}))
 	}
 	selection, err := self.selectionByFile(lines)
 	if err != nil {

pkg/i18n/english.go

@@ -750,6 +750,7 @@ type TranslationSet struct {
 	SelectionNotFoundInDiffToast          string
 	DiffLinesFileNotFound                 string
 	DiffLinesNotFound                     string
+	CommitNotInCommitsPanel               string
 	NoFilesStagedTitle                    string
 	NoFilesStagedPrompt                   string
 	BranchNotFoundTitle                   string
@@ -1942,6 +1943,7 @@ func EnglishTranslationSet() *TranslationSet {
 		SelectionNotFoundInDiffToast:             "Nothing in the selection could be found in the diff",
 		DiffLinesFileNotFound:                    "Can't find the file '{{.path}}' that the selected lines belong to",
 		DiffLinesNotFound:                        "Can't find the selected lines in the diff of '{{.path}}'",
+		CommitNotInCommitsPanel:                  "Can't find commit {{.hash}} in the commits panel",
 		CommitPrefixPatternError:                 "Error in commitPrefix pattern",
 		NoFilesStagedTitle:                       "No files staged",
 		NoFilesStagedPrompt:                      "You have not staged any files. Commit all files?",