UPDATE
I think this may be a duplicate of git whitespace woes
END UPDATE
Unless you've done some customization into your git post-commit hooks or to your git config core.whitespace settings (thanks for pointing that out jesse), git does not translate spaces to tabs or vice versa or anything like that. The changes are usually results of your environment/files and your merge activities.
Having dealt with this in various forms, my guess is that some of your peers are using tabs and some are using spaces. People will have flame wars all day long on whether you should use tabs or spaces, and depending on the day and language I hop sides. That's not something I will get into here.. However just about every rational participant in said war would agree that regardless of which you pick, you need to pick one and use it consistently throughout a project.
Most reasonable editors allow you to control whether tabs or spaces should be inserted, and I know XCode is no stranger to this.
I would suggest showing the whitespace or using briefly an editor which allows you to see the whitespace to see what the difference actually is. Seperately, after you correct a whole file you could also just use git diff -w -b to suppress white space changes. That said you should really figure out what the heck is causing your whitespace to go nuts on a rebase because that suggests to me most likely git is confused by mixed usages, and multiple people correcting it at multiple points with different resolutions.
Good luck to you, it's never easy, and someone is always unhappy when their cheese gets moved.