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