Merge branch 'ps/report-failure-from-git-stash'

"git stash" sometimes was silent even when it failed due to
unwritable index file, which has been corrected.

* ps/report-failure-from-git-stash:
  builtin/stash: report failure to write to index

diff --cc builtin/stash.c
Simple merge

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