match: make glob '**/' match the empty string

Previously, a glob pattern of the form 'foo/**/bar' would match 'foo/a/bar' but
not 'foo/bar'. That was because the '**' in 'foo/**/bar' would be translated to
'.*', making the final regex pattern 'foo/.*/bar'. That pattern doesn't match
the string 'foo/bar'.

This is a bug because the '**/' glob matches the empty string in standard Unix
shells like bash and zsh.

Fix that by making the ending '/' optional if an empty string can be matched.

--HG--
branch : stable