Update translations for polish language (#3389)

- **PR Description**

Hi, I'm happy using `lazygit` from quite a while. I've decided to give
back to community fraction of my time. I'm native polish speaker, and
seen that polish translation have some gaps.

I've added missing translation for polish language and improved existing
one. I've followed technical jargon used on
https://git-scm.com/book/pl/v2/
 
For easier diff comparison and feature maintenance, I've ordered keys to
be in same order as those in `english.go`

Author: stefanhaller