Merge branch 'tg/stash-refresh-index'

"git stash" learned to write refreshed index back to disk.

* tg/stash-refresh-index:
  stash: make sure to write refreshed cache
  merge: use refresh_and_write_cache
  factor out refresh_and_write_cache function

diff --cc builtin/am.c
Simple merge

diff --cc builtin/merge.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc read-cache.c
Simple merge

diff --cc t/t3903-stash.sh
Simple merge