diff: fix 2-way diff panes not being shown side by side eee7674cef41 had the side effect of setting overflow styling on the DOM elements used by mergely. That confused it's computation of sizes on some browsers, scroll bars got added, and the panes were re-flowed to not be side-by-side. To fix that, add an exception that allow overflow-x to be set to visible.

[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