more _git improvements?

[Wayne Davison <wayned@users.sourceforge.net>]

I was looking at the git alias support in the current _git completion,
and it appears to only be used to list alias words when completing a
subcommand, but has no effect when completing options after an alias.
For instance, I have a git for b=branch in my .gitconfig file, and if I
press TAB after "git b" it is listed as an alias.  However if I type an
extra space and a dash before pressing TAB ("git b -"), I don't get any
completions.  Can the completion system substitute the alias text and
show me the completions for "git branch -" (presumably without actually
changing the "b" into "branch" on the command-line)?

One other thing I noticed is that the "config" subcommand appears twice:
once in useful_commands, and once in ancillary_commands.  This results
in some completing confusion when completing "conf".  I chose to just
dump one of them (I dumped the "useful" one), and that makes it work
better in my limited testing (but I'm not sure it's right).

Finally, if someone checks in some changes, there is a "thee" that
should be "the" in the "gc" command:

    'gc:cleanup unnecessary files and optimize thee local repository'

I could check-in a fix for that, but I figure it can wait for a more
substantial change (if one is coming).

..wayne..