fix unicode display

[kanlac]

• I've read Contribution guide
• I've tested everything that doesn't relate to tcell.Screen API

Close #76.

Compared to adding extra blank runes (#77), I think a more reasonable way is to fix the implementation of GetCell.

[viktomas]

Thank you @kanlac 🙏

The implementation makes sense to me, can you please add to your PR description a test case that I can try with the current version and then with your PR to see this change in action?

Once I manually re-test this, the PR is good to go

[kanlac]

Sure, it's easy to reproduce. You can create a file or directory with Unicode characters, for example mkdir 测试目录, and then run godu -l 0. You will see that the second and fourth characters are missing.

After this PR change, the file names will display correctly:

[viktomas]

Thank you @kanlac, I tested this locally, and it works well 👍