Ensure that when examining the working directory for checkout that the
mode is correctly simplified.  Git only pays attention to whether a file
is executable or not.  When examining a working directory, we should
coalesce modes in the working directory to either `0755` (indicating
that a file is executable) or `0644` (indicating that it is not).

Test this by giving the file an exotic mode, and ensuring that when
checkout out a branch that changes the file's contents, that we do not
have a checkout conflict.