Fix multibyte handling

[mattn]

multibyte characters shouldn't be treated length with strlen. it should be used with strdisplaywidth.
For example, あ is 3 bytes in utf-8, but 2 cells width. So it should use strdisplaywidth().

For example:

This should be:

But currently:

This change fix the issue.

[greduan]

@Lokaltog I 👍 this. Awesome addition!

Although I don't experience this problem, I definitely love these kind of bug fixes. 😉

[lilydjwg]

Hi mattn, I'd like this bug to be fixed, but it dosen't work quite correct for me. Look the English words in the blue circles below:

[mattn]

@lilydjwg What is value of ambiwidth ?

[lilydjwg]

@mattn double. And I can see some highlighting issues in your screenshot too. Maybe related to +conceal feature? My conceallevel=2 and concealcursor=nc in that window.

[mattn]

@lilydjwg I think my changes does not depend on the bug. Probably, it occur without my changes?

[lilydjwg]

@mattn Yes!

[mattn]

@lilydjwg Thank you, I guess you've better to file new issue. :)

[haya14busa]

I included multibyte bug fix with some modifications and pushed to Lokaltog/vim-easymotion repository.

Thanks!