Previously we could try to remove the branch we are on.
Of course, we have a test-case elaborate enough to verify we don't
destroy changes in submodules accidentally. Therefore I am confident
that this implementation is correct.

Fixes #49

The latter happened as now BadName is thrown, instead of BadObject.
Changes.rst was marked accordingly

Fixes #50

Previously, the implementation would gladly reset new commits in submodules,
and/or reset a dirty working tree.

Now the new force_reset/force flag has to be specified explicitly to get back
to the old behaviour.

All submodule tests except for one are working.

More to come, especially when it's about strings

All performance tests still print to stderr, but do so in a py3 compatible way

There is more work to do though, as many imports are still incorrect.
Also, there are still print statements

Commandline was
autopep8 -j 8 --max-line-length 120 --in-place --recursive --exclude "*gitdb*,*async*" git/

W291 trailing whitespace

E201 whitespace after '('
E202 whitespace before ')'
E203 whitespace before ':'
E225 missing whitespace around operator
E226 missing whitespace around arithmetic operator
E227 missing whitespace around bitwise or shift operator
E228 missing whitespace around modulo operator
E231 missing whitespace after ','
E241 multiple spaces after ','
E251 unexpected spaces around keyword / parameter equals