Staging OSTree sources with very strict permissions fails

Summary

When trying to build an element whose sources contain files without read/write permissions (e.g., 0111), staging sources fails. This has been observed with an OSTree source, however, the issue may be independent of the source plugin.

Steps to reproduce

Run bst build centos.bst in the attached project repro.tar.gz, from #429 (comment 104869069).

What is the current bug behavior?

Staging sources fails with a BUG message.

What is the expected correct behavior?

Staging sources succeeds.

Relevant logs and/or screenshots

$1758155

Possible fixes

git bisect result:

849724ce197850afb4094a5484a5aabbb8a192e4 is the first bad commit
commit 849724ce197850afb4094a5484a5aabbb8a192e4
Author: Jim MacArthur <jim.macarthur@codethink.co.uk>
Date:   Tue May 8 16:19:22 2018 +0100

    element.py: Conversion to use virtual directories

Other relevant information

This is a regression in BuildStream master (1.3). It works in BuildStream 1.2.