We work on a project hosted on code.google.com with several friends. It's open source so all the source files are available for general public. However, I'd like to contribute some files (mainly for testing purposes) with confidential information. Is it possible to hide one file contents, i.e provide it only for other committers of the project?
No.
code.google.com is for open source projects only. All stuff that you upload there is supposed to be "open" to everyone.