[RFC/PATCH 2/4] rev-cache: non-commit object caching + tests

Part of the revision-cache patch series, this patch adds preliminary non-commit object support.  Non-start commits are diff'ed against parents to produce a list of objects 'unique' to that commit, which are then added to the pending objects list in traversal.

Signed-off-by: Nick Edelen <redacted>

---
 rev-cache.c               |  204 ++++++++++++++++++++++++++++++++++++++++++++-
 t/t6015-rev-cache-list.sh |   28 ++++++
 2 files changed, 230 insertions(+), 2 deletions(-)