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.
tags: read tag info into a sorted dict (rather than into a regular dict)
This lets us preserve the original tag order (as specified on the .hgtags file
that is being read). This will be useful to preserve the tag order when saving
the result of a successful automated .hgtags merge (which will be introduced on
a future patch).
There shouldn't be much impact on performance because the sortdict that
_readtags returns is then used to update the alltags regular dict (which
remains a regular dict).