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`.
running: init test1
result: None
running: add foo
result: 0
running: commit -m commit1 -d 2000-01-01 foo
result: None
running: commit -m commit2 -d 2000-01-02 foo
result: None
running: log -r 0
changeset: 0:0e4634943879
user: test
date: Sat Jan 01 00:00:00 2000 +0000
summary: commit1
result: None
running: log -r tip
changeset: 1:45589e459b2e
tag: tip
user: test
date: Sun Jan 02 00:00:00 2000 +0000
summary: commit2
result: None