When an error occurs during completions, a message is displayed on
the status line. If this message is longer than the width of the
current window, Vim will prompt the user to press enter or type a
command to hide the message, interrupting user workflow. We prevent
that by truncating the message to window width.
Merge PostMultiLineNotice, EchoText, and EchoTextVimWidth functions
into PostVimMessage.
Moved File parse request handling and diagnostic extraction flow into
python to simplify flow and allow easier addition of new parse request
handlers such as semantic highlighter.
Refactored base_test to patch separate vimsupport functions instead of
the whole module, and interfering the test results afterwards.
Added new tests for diagnostic sign place/unplace and error/warning
count extraction API.
assert_has_calls is not enough to check if a call didn't raise a
warning. We also need to check the number of calls. This is done by
creating a subclass of MagicMock implementing the
assert_has_exact_calls method.
