Updated _git completion (not attached)

[Nikolai Weibull <now@bitwi.se>]

OK.  I’m not doing this again.  This is perhaps the most boring thing
I’ve ever done (since writing _git the first time).  6037 lines of
pure boring.  167 remaining TODOs.

https://github.com/now/zsh

I don’t know where we go from here.  It’s a ridiculous history,
spanning almost one and a half years (though I took a rather long
hiatus during that time).  I reordered all functions to match the
order in the manual.  I reordered all option specifications to match
the order in the manual.  Perhaps the easiest thing to do is simply
throw away the whole history and simply overwrite the current _git
with this one.

I’d appreciate comments and updates to be provided as soon as
possible, so that I can review them while I still have most of this
internalized.

Anyway, this is for git (at or about) 1.7.3.5.

A note on the discussion of not using git ls-files for generating file
names together with _multi_parts: I tried the simple (e:…) filter for
_files but it turned out to be SLOW.  Perhaps someone else can look
into this and see if they can come up with a working solution.

I also updated the _git_commands function to use tags.  Can someone
please explain how I ignore plumbing-sync-helper-commands and
plumbing-internal-helper-commands?  I tried using the tag-order style,
but it wouldn’t take.  I’m obviously doing something wrong.  I also
removed support for user-commands here.  Perhaps they should be added
back in.