Merge branch 'tr/cache-tree'

* tr/cache-tree:
reset: update cache-tree data when appropriate
commit: write cache-tree data when writing index anyway
Refactor cache_tree_update idiom from commit
Test the current state of the cache-tree optimization
Add test-scrap-cache-tree

diff --cc .gitignore
Simple merge

diff --cc Makefile
Simple merge

diff --cc builtin/commit.c
Simple merge

diff --cc merge-recursive.c
Simple merge