Let a stdin filter be told the width of the diff it renders

A stdin filter finds out how wide to lay out its rendering by asking the
terminal. That is one of the two jobs the pty around a render does.
Rendering through a pipe instead, as Windows is about to do, leaves the
renderer to pick a width of its own, and a side-by-side rendering comes
out at the wrong size.

Offer the width as the {{width}} template variable, so that a
configuration can name it on the command line where the renderer can no
longer ask for it. {{columnWidth}} is derived from the same number.

This is only needed by diff renderers which don't support the COLUMNS
variable, which we will set later in this branch; delta and
diff-so-fancy both do in their latest versions, so we don't document
this.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

pkg/config/diff_renderer_config_manager.go

@@ -67,6 +67,7 @@ func (self *DiffRendererConfigManager) GetStdinFilterCommand(width int) string {
 	}
 
 	templateValues := map[string]string{
+		"width":       strconv.Itoa(width),
 		"columnWidth": strconv.Itoa(width/2 - 6),
 	}
 

pkg/config/diff_renderer_config_manager_test.go

@@ -63,6 +63,56 @@ func TestCurrentDiffRendererName(t *testing.T) {
 	}
 }
 
+func TestGetStdinFilterCommand(t *testing.T) {
+	scenarios := []struct {
+		name               string
+		diffRendererConfig DiffRendererConfig
+		width              int
+		expected           string
+	}{
+		{
+			name:               "a command without template variables is passed through",
+			diffRendererConfig: DiffRendererConfig{Command: "delta --paging=never"},
+			width:              120,
+			expected:           "delta --paging=never",
+		},
+		{
+			name:               "the width the diff is rendered at",
+			diffRendererConfig: DiffRendererConfig{Command: "delta --width={{width}}"},
+			width:              120,
+			expected:           "delta --width=120",
+		},
+		{
+			name:               "the width of one side of a side-by-side rendering",
+			diffRendererConfig: DiffRendererConfig{Command: "ydiff -p cat -w {{columnWidth}}"},
+			width:              120,
+			expected:           "ydiff -p cat -w 54",
+		},
+		{
+			name:               "a template variable can also be written with a leading dot",
+			diffRendererConfig: DiffRendererConfig{Command: "delta --width={{.width}}"},
+			width:              120,
+			expected:           "delta --width=120",
+		},
+		{
+			name:               "nothing is returned for a renderer of another type",
+			diffRendererConfig: DiffRendererConfig{Type: "extDiff", Command: "difft --width={{width}}"},
+			width:              120,
+			expected:           "",
+		},
+	}
+
+	for _, s := range scenarios {
+		t.Run(s.name, func(t *testing.T) {
+			userConfig := &UserConfig{}
+			userConfig.Git.DiffRenderers = []DiffRendererConfig{s.diffRendererConfig}
+			config := NewDiffRendererConfigManager(func() *UserConfig { return userConfig })
+
+			assert.Equal(t, s.expected, config.GetStdinFilterCommand(s.width))
+		})
+	}
+}
+
 func TestCurrentDiffRendererNameWithoutDiffRenderers(t *testing.T) {
 	config := NewDiffRendererConfigManager(func() *UserConfig { return &UserConfig{} })