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

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.

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

Old question (2011), but now there's a shortcut git diff -w which stands for --ignore-all-space

Ignore whitespace when comparing lines. This ignores differences even if one line has whitespace where the other line has none.

I'd agree with Dogbert's answer that it's probably best to just use an alias, but another option is to set the config option diff.external to a wrapper script that calls diff with -b.

This doesn't answer your question exactly, but it's a way to achieve something similar for apply.

From man git-config:

 apply.whitespace
       Tells git apply how to handle whitespaces, in the same way
       as the --whitespace option. See git-apply(1).

So open up your ~/.gitconfig or ./.git/config/ and append

[apply]
   whitespace = nowarn

It might also not let you commit something that only changes whitespace, but I'm sure you can overrule that with some flags.