lei: fix globbing semantics to match end-of-filename
Globs such as `*/foo' should not match `*/foobar'. I noticed
this while adding glob support to public-inbox-clone.
This may subtly break some existing cases, but there aren't many
lei users, yet, and globbing semantics should match what most
other glob-using programs, do...
We'll also make `lei ls-mail-sync' behave more consistently with
`lei ls-external', as far as the basename matching fallback
goes.