Sometimes I'm working on multiple sections of a project at once and I need to commit a subset of the files. I used to pick Commit off the Source menu and uncheck the files I did not want to commit. Doing it that way usually results in files mysteriously unchecking themselves or just not committing.
I found a work around was to select only the files I wanted to commit in the Project Navigator, right/ctrl-click and pick Source/Commit and that only shows those files. I can then compare the files.
Hope this helps!
Xcode 7 git seems to be ignoring my user.name setting as well and is instead using the OSX user name.
Regards, Patrick