parameterise rebase-filter-maint#1

This commit parameterises the rebase-filter-maint script to make it more user friendly. Minimal changes were made to the preset envvars: - `maintainers` was removed - set it as an envvar when calling the script or use e.g. `--maintainers python@` - `minefield` is now set to the current git branch by default (though this can be overwriten using `--minefield` - `debug` is no longer a default option (we can easily call it when we want it now) I've gotta actually get on with getting these ebuilds merged, but it shouldn't be too hard from here to make life easier by setting some default values (so that envvars that we pass in aren't overwritten by what's in the file) and some additional params. No changes to logic aside from the removed preset envvars. All seems to work - I've got a neat pile of filtered commits to start QAing and testing now...