type `O` under folded line in normal mode may make fold color disabled

[kevinhwang91]

Neovim version (nvim -v | head -n1)

master

Operating system/version

ArchLinux

How to reproduce the issue

local function level1()
    local function level2()

    end
end

1. 2ggzczc
2. O

Expected behavior

Actual behavior

[kevinhwang91]

Caused by level1 and level2 both have folded, can't identify level1 have been become line 2 and skip to refresh its virtual text in the decorator.

Solution: Get the text per line and use textLine as a field to compare. But we should use a buf lines model to cache the result, otherwise using vim.api.nvim_buf_get_lines directly will get obvious perf regression.

[kevinhwang91]

fixed.