Problem: The 'statuscolumn' width is being estimated without the
proper context. In particular, this resulted in the fact
that a custom fold column could be included in the estimated
`number_width()`, and doubly added when actually drawing the
statuscolumn due to `win_col_off()` also adding the
`'foldcolumn'` width. Resulting in a status column that is
`'foldcolumn'` cells wider than necessary.
Solution: Estimate 'statuscolumn' width in `get_statuscol_str()` when
a buffer's line count has changed.
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