Updated zsh completion patches

[David Bremner]

Hi Vincent;

Thanks for this patch, it's definitely a big improvement. I had to
make some small adjustments to the install recipes to accomodate the
renaming / removal of notmuch-completion.zsh. A revised version of
your patch with that squashed in follows.

I also added a patch to install the completion files properly on
Debian. That could be squashed in, or left as a seperate patch.

I do have one problem with the completion of tags. It seems the
treatment of tags with :: in them is not consistent.

If I run

   % notmuch tag +foo::bar tag:foo::bar

for some existing tag foo::bar, the second one fails to complete

C-x h yields something like the following

╭─ convex:~ 
╰─% notmuch tag +inbox::notmuch tag:inbox:
tags in context :completion::complete:notmuch::
    argument-rest  (_arguments _notmuch)
tags in context :completion::complete:notmuch:argument-rest:
    normal-argument-rest  (_arguments _notmuch_tag _notmuch_command _arguments _notmuch)
tags in context :completion::complete:notmuch:normal-argument-rest:
    normal-argument-rest  (_message _notmuch_search_term _notmuch_tagging_or_search _arguments _notmuch_tag _notmuch_command _arguments _notmuch)