Add the directory which contains the editor's executable to your PATH variable. (E.g. "C:\Program Files\Sublime Text 3\")
Reboot your computer.
Change the core.editor global Git variable to the name of the editor executable without the extension'.exe' (e.g. git config --global core.editor sublime_text)
That's it!
NOTE: Sublime Text 3 is the editor I used for this example.