Add a new branch to the `testrepo` repository, where the `README` file
has changed to executable.  This branch enables typechange tests between
the new `executable` branch and `master`.

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.

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.

Fixes #4504