zsh_completion: make use of `debuglabelcomplete` command Use `debuglabelcomplete` command when populating labels list, instead of calling `hg` three times: for branches, bookmarks and tags. Do not pass string being completed to `hg debuglabelcomplete` (as `$words[$CURRENT]`), since it breaks `_hg_revrange` completion (`--rev 2.5:2.5.<Tab>`) for no apparent benefit. Also complete `hg view` with labels, not just tags, and drop unused `_hg_tags`.