Merge branch 'jc/fail-stash-to-store-non-stash'

Feeding "git stash store" with a random commit that was not created
by "git stash create" now errors out.

* jc/fail-stash-to-store-non-stash:
  stash: be careful what we store

diff --cc builtin/stash.c
Simple merge

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