Add ScrollOffMargin user config

When set to a non-zero value, views will scroll when the selection gets this
close to the top or bottom of the view.

pkg/config/user_config.go

@@ -31,6 +31,7 @@ type GuiConfig struct {
 	BranchColors              map[string]string  `yaml:"branchColors"`
 	ScrollHeight              int                `yaml:"scrollHeight"`
 	ScrollPastBottom          bool               `yaml:"scrollPastBottom"`
+	ScrollOffMargin           int                `yaml:"scrollOffMargin"`
 	MouseEvents               bool               `yaml:"mouseEvents"`
 	SkipDiscardChangeWarning  bool               `yaml:"skipDiscardChangeWarning"`
 	SkipStashWarning          bool               `yaml:"skipStashWarning"`
@@ -418,6 +419,7 @@ func GetDefaultConfig() *UserConfig {
 		Gui: GuiConfig{
 			ScrollHeight:             2,
 			ScrollPastBottom:         true,
+			ScrollOffMargin:          2,
 			MouseEvents:              true,
 			SkipDiscardChangeWarning: false,
 			SkipStashWarning:         false,