Merge branch 'bc/stash-export-import'

An interchange format for stash entries is defined, and subcommand
of "git stash" to import/export has been added.

* bc/stash-export-import:
  builtin/stash: provide a way to import stashes from a ref
  builtin/stash: provide a way to export stashes to a ref
  builtin/stash: factor out revision parsing into a function
  object-name: make get_oid quietly return an error

diff --cc builtin/stash.c
Simple merge

diff --cc object-name.c
Simple merge

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