Avoid race condition crash on directory deletion

Given the directory structure /a/b/c.txt there is a race condition
in DirectorySnapshot if it reads the list of entries in /a, then
/a/b is deleted, and then it tries to read /a/b. This happens
often in practice when changing between very different branches in
git.

The ideal behaviour would be to report /a/b as not existing in this
case, but we cannot do this without either changing the order of the
walk (from parent-first to parent-last) or significantly increasing
memory usage and copies, so instead we report /a/b as existing but
empty.