we don't need to use difflib to compare files

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>
Merge pull request #17 from ignatenkobrain/bindiff

we don't need to use difflib to compare files
do not remove files on those merge tools, which does not return correct exit code
Allow specification of a custom merge type via an environment variable, $MERGE

inspired by mavit patch
https://github.com/xsuchy/rpmconf/pull/6/files
https://github.com/xsuchy/rpmconf/pull/7
Automatic commit of package [rpmconf] release [1.0.3-1].
add BR rpm-python3

addressing:
F: 23, 0: Unable to import rpm (import-error)
Automatic commit of package [rpmconf] release [1.0.4-1].