Merge branch 'jk/log-decorate-optim'

Merge branch 'jk/log-decorate-optim'

Optimize "git log" for cases where we wasted cycles to load ref
decoration data that may not be needed.

* jk/log-decorate-optim:
  load_ref_decorations(): fix decoration with tags
  add_ref_decoration(): rename s/type/deco_type/
  load_ref_decorations(): avoid parsing non-tag objects
  object.h: add lookup_object_by_type() function
  object.h: expand docstring for lookup_unknown_object()
  log: avoid loading decorations for userformats that don't need it
  pretty.h: update and expand docstring for userformat_find_requirements()