Expose SelectedCommit to custom commands, deprecate Selected{Local,Reflog,Sub}Commit

SelectedCommit is context-dependent and points to SelectedLocalCommit,
SelectedReflogCommit, or SelectedSubCommit depending on which panel is active.

If none of these panels is active, it returns the selected local commit, which
is probably the most useful default (e.g. when defining custom commands for the
Files panel).

pkg/gui/types/context.go

@@ -284,6 +284,7 @@ type IContextMgr interface {
 	CurrentSide() Context
 	CurrentPopup() []Context
 	IsCurrent(c Context) bool
+	IsCurrentOrParent(c Context) bool
 	ForEach(func(Context))
 	AllList() []IListContext
 	AllFilterable() []IFilterableContext