functions: apply diff-filter to changed_repos

changed_aports uses --diff-filter=ACMR so that it only get aports with relevant changes. Removed aports for example do not need to be built.

But changed_repos omitted this filter. This lead to discrepancies between these two functions. This specifically resulted into issues when packages were moved.

changed_repos would for example return 'main', when a package would be moved from main to community. changed_aports then would not report any package to be changed. But the way the amount of changed packages are counted, it would report '1', even though no packages have been built for this repo.

This in turn results build_start going negative, which is used as a start range for sed, which then tries to interpret it as an argument which fails.

Fix this by applying the same filter to changed_repos as changed_aports uses.