Extract a ListRenderer struct

I'm doing this not so much because it's a great abstraction, but just because it
will make it much easier to write tests for it.

pkg/gui/context/list_renderer.go

@@ -0,0 +1,27 @@
+package context
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/gui/types"
+	"github.com/jesseduffield/lazygit/pkg/utils"
+)
+
+type ListRenderer struct {
+	list              types.IList
+	getDisplayStrings func(startIdx int, endIdx int) [][]string
+	// Alignment for each column. If nil, the default is left alignment
+	getColumnAlignments func() []utils.Alignment
+}
+
+func (self *ListRenderer) GetList() types.IList {
+	return self.list
+}
+
+func (self *ListRenderer) renderLines(startIdx int, endIdx int) string {
+	var columnAlignments []utils.Alignment
+	if self.getColumnAlignments != nil {
+		columnAlignments = self.getColumnAlignments()
+	}
+	return utils.RenderDisplayStrings(
+		self.getDisplayStrings(startIdx, utils.Min(endIdx, self.list.Len())),
+		columnAlignments)
+}