model: refactor and simplify _get_instance _get_instance is a BaseModel method, but never uses "self". Instead it takes a class argument, which indicates that it's better suited as a classmethod on said classes. Also rename to something more descriptive, remove leading underscore since it's not a private API, and refactor for readability.

[pytest]
# only look for tests in kallithea/tests
python_files = kallithea/tests/**/test_*.py
addopts =
    # --verbose
    # show extra test summary info as specified by chars (f)ailed, (E)error, (s)skipped, (x)failed, (X)passed, (w)warnings.
    -rfEsxXw
    # Shorter scrollbacks; less stuff to scroll through
    --tb=short