* Refactored HGRepoManager to better support multiple tags:
- `.get_tag` replaced by `.get_tags`, which returns a set of tags
for a specific revision. This is currently a set because mercurial
does not retain any meaningful order of the tags.
- `.get_parent_tag` replaced by `.get_parent_tags`.
- added `.get_parents` which returns the revision(s) of the specified
* Removed support for older versions of mercurial (LegacyLibraryManager).
* The subprocess manager is now the default. The HGTOOLS_FORCE_CMD variable
no longer has any effect on hgtools.