git config --global core.editor "'C:/Program Files/Notepad++/notepad++.exe' -multiInst -notabbar -nosession -noPlugin"
Or, for 64-bit Windows and a 32-bit install of Notepad++:
git config --global core.editor "'C:/Program Files (x86)/Notepad++/notepad++.exe' -multiInst -notabbar -nosession -noPlugin"
Or, the following can be issued on the command line on either 32-bit or 64-bit Windows. It will pull the location of notepad++.exe from the registry and configure git to use it automatically:
FOR /F "usebackq tokens=2*" %A IN (`REG QUERY "HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\App Paths\notepad++.exe" /ve`) DO git config --global core.editor "'%B' -multiInst -notabbar -nosession -noPlugin"
If you wish to place the above from a .BAT or .CMD file, you must replace %A with %%A and %B with %%B