Disable staging and unstaging lines or hunks when the diff context size is 0 (#4235)

- **PR Description**

Git diff and patch doesn't work reliably with a context size of 0, so
disable it in this case (and discarding changes as well). Magit does the
same, see magit/magit#4222.

Staging entire files by pressing space in the Files panel is still
possible, of course.

Fixes #4233.

Author: stefanhaller

pkg/gui/controllers/staging_controller.go

@@ -1,11 +1,13 @@
 package controllers
 
 import (
+	"fmt"
 	"strings"
 
 	"github.com/jesseduffield/gocui"
 	"github.com/jesseduffield/lazygit/pkg/commands/git_commands"
 	"github.com/jesseduffield/lazygit/pkg/commands/patch"
+	"github.com/jesseduffield/lazygit/pkg/gui/keybindings"
 	"github.com/jesseduffield/lazygit/pkg/gui/types"
 )
 
@@ -185,10 +187,20 @@ func (self *StagingController) TogglePanel() error {
 }
 
 func (self *StagingController) ToggleStaged() error {
+	if self.c.AppState.DiffContextSize == 0 {
+		return fmt.Errorf(self.c.Tr.Actions.NotEnoughContextToStage,
+			keybindings.Label(self.c.UserConfig().Keybinding.Universal.IncreaseContextInDiffView))
+	}
+
 	return self.applySelectionAndRefresh(self.staged)
 }
 
 func (self *StagingController) DiscardSelection() error {
+	if self.c.AppState.DiffContextSize == 0 {
+		return fmt.Errorf(self.c.Tr.Actions.NotEnoughContextToDiscard,
+			keybindings.Label(self.c.UserConfig().Keybinding.Universal.IncreaseContextInDiffView))
+	}
+
 	reset := func() error { return self.applySelectionAndRefresh(true) }
 
 	if !self.staged && !self.c.UserConfig().Gui.SkipDiscardChangeWarning {

pkg/i18n/english.go

@@ -926,6 +926,8 @@ type Actions struct {
 	UnstageFile                       string
 	UnstageAllFiles                   string
 	StageAllFiles                     string
+	NotEnoughContextToStage           string
+	NotEnoughContextToDiscard         string
 	IgnoreExcludeFile                 string
 	IgnoreFileErr                     string
 	ExcludeFile                       string
@@ -1913,6 +1915,8 @@ func EnglishTranslationSet() *TranslationSet {
 			UnstageFile:                     "Unstage file",
 			UnstageAllFiles:                 "Unstage all files",
 			StageAllFiles:                   "Stage all files",
+			NotEnoughContextToStage:         "Staging or unstaging changes is not possible with a diff context size of 0. Increase the context using '%s'.",
+			NotEnoughContextToDiscard:       "Discarding changes is not possible with a diff context size of 0. Increase the context using '%s'.",
 			IgnoreExcludeFile:               "Ignore or exclude file",
 			IgnoreFileErr:                   "Cannot ignore .gitignore",
 			ExcludeFile:                     "Exclude file",