add current git-annex-git package

I am not (yet) adopting this one, but as I update the other two variants
(git-annex and git-annex-bin) some of the fixes will inevitably be
relevant here and having all three in my repos will make it easy to keep
track of changes and submit them to the current maintainer.
update git-annex-git credits
round out mutual conflicts with other two pakcage variants
switch to using makepkg's built in repository handler

A lot of the logic needed to deal with VCS repos during build is
mainlined as part of the package builder. Using this functionality
instead of rolling a custom one allows the build process to respect
various flags such as whether or not to keep the checkout for use next
time, etc.