Update translations for polish language

[undg]

• 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

• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• Docs (specifically docs/Config.md) have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[stefanhaller]

@undg Thanks for the contribution!

I don't speak Polish, so I can't judge the quality of these translations, but I'm sure it's better than before. 😄

I had to resolve conflicts when rebasing onto master. Gonna merge now, thanks again!