Truncate invisible lines before storing ref-tests

Because there is no good way to store invisible lines in a backwards-
and forwards-compatible way, they buffer now gets truncated before
dumping the state of a grid when creating a ref-test.

This involved a few workaround of which a few required adding additional
methods which are only used in ref-tests, these should be minimal
though.

Since this required the creation of a truncation method anyways, some
logic has been added which automatically truncates the invisible buffer
when there are more than X (set to 100) invisible lines. This should not
impact performance because it rarely occurs, but it could save a bit of
memory when the history size is shrunk during runtime (see #1293).

This also adds an optional `config.json` file to the ref-test output
where it is possible to manually specify variables which should override
config defaults, this has been used only for history_size so far.

Creating a new ref-test does also still work, so there was no regression
here, if history size is altered, the config.json just has to be created
manually with the content `{"history_size":HIST_SIZE}`, where
`HIST_SIZE` is the desired history size.

tests/ref/grid_reset/config.json

@@ -0,0 +1 @@
+{"history_size":100}