Don't check file permissions on Windows

There isn't really an executable bit on Windows, and depending on the git
implementation, and the setting of the core.filemode attribute the mode
may be either 755 or 644.
The --allow-empty-message option was added in git 1.7.0.5.

This test does not only fail for me, but hangs indefinitely (with cygwin
git 1.6.1.2).  Not sure why it hangs, but since it would fail if it
didn't hang, I didn't investigate further.