A `--setup` step in `git filter-branch` makes it much easier to
define the initial values of variables used in the real filters.
Also sourcing/defining utility functions here instead of
`--env-filter` improves performance and minimizes clogging the
output in case of errors.
Signed-off-by: Andreas Heiduk <asheiduk@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>