merge-ort: add data structures for in-memory caching of rename detection

merge-ort: add data structures for in-memory caching of rename detection

When there are many renames between the old base of a series of commits
and the new base for a series of commits, the sequence of merges
employed to transplant those commits (from a cherry-pick or rebase
operation) will repeatedly detect the exact same renames.  This is
wasted effort.

Add data structures which will be used to cache rename detection
results, along with the initialization and deallocation of these data
structures.  Future commits will populate these caches, detect the
appropriate circumstances when they can be used, and employ them to
avoid re-detecting the same renames repeatedly.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>