Per release notes:

https://github.com/rpm-software-management/mock/wiki/Release-Notes-2.5

mock

• because of the mock-filesystem change, we need to enforce upgrade of the old mock-core-configs package
• set the DNF user_agent in dnf.conf (msuchy@redhat.com)
• introduce mock-filesystem subpackage (msuchy@redhat.com)
• add showrc plugin to record the output of rpm --showrc (riehecky@fnal.gov)
• document which packages we need in buildroot (msuchy@redhat.com)
• macros without leading '%' like config_opts['macros']['macroname'] work fine again (issue#605)

mock-core-cofnigs

• provide the Fedora ELN mock configuration
• some adjustments were done for the new mock-filesystem package

https://github.com/rpm-software-management/mock/wiki/Release-Notes-2.6

the --recurse option implies --continue
fix --chain --continue option
fail when --continue/--recurse is used without --chain
fix _copy_config() for broken symlinks in dst= (#1878924)
auto-download the source RPMs from web with --rebuild
handle exceptions from command_parse() method
fail verbosely for --chain & --resultdir combination
allow using -a|--addrepo with /absolute/path/argument
add support for -a/--addrepo in normal --rebuild mode
use systemd-nspawn --resolv-conf=off
create /etc/localtime as symlink even with isolation=simple (msuchy@redhat.com)
dump the reason for particular package build fail in --chain
raise PkgError when the source RPM can not be installed