Just configure different editor. See How can I set up an editor to work with Git on Windows?.
The answer there is a bit longish; the most relevant command is
git config --global core.editor "'C:/Program Files/Notepad++/notepad++.exe' -multiInst -notabbar -nosession -noPlugin"
The editor command has to be in single quotes if it contains a space (as it usually does). The alternate workarounds are using DOS path (C:/PROGRA~1/Notepa~1/...
or somesuch) or creating a wrapper script in some path that does not have spaces.
In either case, use forward slashes for path separator.
You don't want to use plain notepad though. At least notepad++ or such that can handle Unix newlines, encodings and such.