Merge branch 'tr/cache-tree' into maint-1.7.8

* tr/cache-tree:
t0090: be prepared that 'wc -l' writes leading blanks
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 Makefile
Simple merge

diff --cc builtin/commit.c
Simple merge