Merge branch 'bp/post-index-change-hook'

A new hook "post-index-change" is called when the on-disk index
file changes, which can help e.g. a virtualized working tree
implementation.

* bp/post-index-change-hook:
  read-cache: add post-index-change hook

diff --cc Documentation/githooks.txt
Simple merge

diff --cc builtin/reset.c
Simple merge

diff --cc builtin/update-index.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc read-cache.c
Simple merge

diff --cc unpack-trees.c
Simple merge