fix: calculate blob line without strdisplaywidth

[aileot]

This is a workaround for the error "E976: using Blob as a String" on strdisplaywidth.

[ribru17]

Hoping this can be merged soon! This issue is annoying when opening the cmdline in binary files

[aileot]

Sorry for the late reply. I've missed your review. I agree with you, but the magic number should be replaced.

[Bekaboo]

I believe using vim.fn.getline() instead of vim.api.nvim_get_current_line() solves this issue. Can you confirm?

[aileot]

Yes, I can confirm it. It appears reasonable that vim.fn.getline() should have higher compatibility with the irreplaceable endpoint vim.fn.strdisplaywidth() since both are native Vim functions.