Prior to this change, a glob like `**/file.txt` would only match
`file.txt` in subdirectories; the `**` must match at least one directory.
This is historical behavior.
With this change we move a little closer to bash's implementation by
allowing a literal `**` segment to match in the current directory. That
is, `**/foo` will match both `foo` and `bar/foo`, while `b**/foo` will
only match `bar/foo`.
Fixes#7222.