user option magit-git-global-arguments
The arguments set here are used every time the git executable is run as a subprocess. They are placed right after the executable itself and before the git command - as in
git HERE... COMMAND REST. For valid arguments see the git(1) manpage.
Be careful what you add here, especially if you are using Tramp to connect to servers with ancient Git versions. Never remove anything that is part of the default value, unless you really know what you are doing. And think very hard before adding something; it will be used every time Magit runs Git for any purpose.