Okay, I experimented a bit and found out that Windows Git commands actually need UNIX variables like LC_ALL in order to display Polish (or other UTF-8 characters) correctly. Just try this command:
set LC_ALL=C.UTF-8
Then enjoy the result. Here is what happened on my console (font “Consolas”, no chcp necessary):

Update:
- Well, in order for Windows commands like
type(display file on console) to work correctly, you do needchcp 65001. - And if you prefer commands from Git Bash like
catyou profit from the aforementionedset LC_ALL=C.UTF-8.

Update 2: How to make the changes permanent
As user mono blaine said, create an environment variable LC_ALL and assign it the value C.UTF-8, either globally or for your own user profile only (sorry for the German screenshot):

Next time you open a command processor console (cmd.exe) you should see the variable value when issuing the command echo %LC_ALL%. In PowerShell you should see it when issuing $env:LC_ALL.
The simplest way to make the UTF-8 code page permanent ist to open regeedit and add a new value named Autorun of type string to section
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Command Processor and assign it the value chcp 65001.

Henceforth, this command will be executed each time you open a new cmd.exe console. You even see its output in the new window: “Aktive Codepage: 65001.” (or similar in your respective language).
Oh, by the way: In order to display a UTF-8 encoded file correctly in PowerShell you can use Get-Content -encoding UTF8 file.txt or cat -encoding UTF8 file.txt (cat being an alias for Get-Content in PowerShell).