Xcode, even as new as v4.3 sometimes inserts trailing whitespace in edited source files. For example, when you select all lines and select menu Editor->Structure->Re-Indent to reformat the indentations. This can cause unnecessary changes to repository, when actually nothing was changed.
To solve the problem, you can use a git pre commit hook.
1. Set your git repository to highlight trailing spaces in diffs:
git config core.whitespace trailing-space
2. Add this script to run before every commit by saving it in .git/hooks/pre-commit

3. Make it executable:
chmod a+x .git/hooks/pre-commit
Notes:
The above script works even when your repository has files or folders with spaces in them.
There used to be an Xcode plugin that automatically removes them, but with Xcode 4 it requires manual invocation.