How to make git diff --ignore-space-change the default

According to the Git Config manual, there's no such option. Your only option is to make an alias.

http://git-scm.com/docs/git-config

You could use git alias or bash alias if you are using shell-available OS.

1. git alias : Run this command to add alias:

   git config --global alias.dfw 'diff --ignore-space-change'

   --ignore-space-change can be abbreviated to -w
   to apply the alias using: git dfw

2. bash alias : Run this command to add bash alias:

   echo "alias gitdfw='git diff --ignore-space-change'">>~/.profile

   Open a new terminal and you can directly run gitdfw to achieve the same.