Add two tests for filemode.

The first ensures that `core.filemode=true` is honored: if we have
changed the filemode such that a file that _was_ executable (mode 0755)
is now executable (mode 0644) and we go to check out a branch that has
otherwise changed the contents of the file, then we should raise a
checkout conflict for that file.

The second ensures that `core.filemode=false` is honored: in the same
situation, we set a file that was executable to be non-executable, and
check out the branch that changes the contents of the file.  However,
since `core.filemode` is false, we do not detect the filemode change.

We run these tests on both operating systems that obey `core.filemode`
(eg, POSIX) and those that have no conception of filemode (eg, Win32).
This ensures that `core.filemode` is always honored, as it is a cache of
the underlying filesystem's settings.  This ensures that we do not
make assumptions based on the operating system, and honor the
configuration setting even if it were misconfigured.