Merge branch 'jc/index-update-if-able' into maint

* jc/index-update-if-able:
  update $GIT_INDEX_FILE when there are racily clean entries
  diff/status: refactor opportunistic index update

diff --cc builtin/commit.c
Simple merge

diff --cc builtin/diff.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc read-cache.c
Simple merge