git: Add a wrapper for git-{upload,receive}-pack

This wrapper takes care of autocreating repositories and prepending the
/data/vcs/git base path, so git access through SSH can happen using
relative urls.
apt: Add backports to sources.list
Merge branch 'template' of git://github.com/daenney/Tika into login
apt: Use pinning instead of Apt::Default-Release

Setting a default release is useful when two archives (e.g. stable and
testing) are configured, but once a third archive (e.g. unstable) is
added, this gets messy. All no-default archives get chunked together
with the same priority, so once you install a version from any
non-default archive, it will happily upgrade to the most recent version
from another archive.

By giving each archive a separate priority, this shouldn't happen. Since
the version numbers in the archives typically increment in the reverse
order of the priorities, this should do the right thing in most
circumstances.
Merge branch 'template' of git://github.com/daenney/Tika into login