[PATCH 0/5] some "log --decorate" optimizations

I was recently pointed to a repository where:

  git log --format="%H %P" old..new

was abysmally slow, even though old..new only had a few commits, because load_ref_decorations() takes a long time (and kicks in due to the auto-decorate behavior). Leaving aside that "rev-list" would probably be a better tool here, it seems there's some low-hanging fruit:

  - a user-format that doesn't show decorations doesn't need to load
    them (fixed in patch 2 below)

  - we're pretty eager to parse the objects at the tip of each ref
    (fixed in patch 5 below)

The other commits are just cleanups and preparatory refactors.

  [1/5]: pretty.h: update and expand docstring for userformat_find_requirements()
  [2/5]: log: avoid loading decorations for userformats that don't need it
  [3/5]: object.h: expand docstring for lookup_unknown_object()
  [4/5]: object.h: add lookup_object_by_type() function
  [5/5]: load_ref_decorations(): avoid parsing non-tag objects

 builtin/log.c |  3 +++
 log-tree.c    |  6 ++++--
 object.c      | 18 ++++++++++++++++++
 object.h      | 20 +++++++++++++++++++-
 pretty.c      |  4 ++++
 pretty.h      |  8 ++++++--
 reachable.c   | 18 ------------------
 7 files changed, 54 insertions(+), 23 deletions(-)

-Peff