Currently "make debian-snapshot" will include the performance corpus
tarball in the source package, which slows things down and wastes disk
space. tar-ignore is needed twice to keep the default exclude rules
(e.g. to exclude .git)
This generates a seperate notmuch-0.x.debian.tar.gz containing
./debian.
In the initial release this is redundant, but for Debian only updates
between releases, this allows updating the contents of ./debian, and
using the rest of the release tarball.
