These files were for ideas and explorations of solutions. We now have a
working implementation independent of them.

Since it doesn't really make sense to take multiple refs, don't allow
more than one. We can only diff between two trees, and one is fixed as
the workdir.

Also learned that to get an exit code from a `system()` call in Perl,
you have to shift `$?` by 8
(https://perldoc.perl.org/functions/system.html).

Turns out `refname_to_id()` wasn't the right function to get a
reference/oid/object from a ref name. Use `revparse_single()` instead to
get a usable reference object.

Add a test for custom diff bases.

Update the expectation to include the file name and line number plus the
exact line content as printed by the command.

Remove the extra newline from the output so that now only a single
newline is printed.

Test that removed TODO lines don't appear in the command's output.

This currently gives the following error:

    thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', src/main.rs:39:43
    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
    thread 'main' panicked at 'Box<Any>', $HOME/.cargo/registry/src/github.com-1ecc6299db9ec823/git2-0.13.11/src/panic.rs:27:9

Need to restore the repo state back to the master branch and remove the
new branch created in this test.

Look for TODO lines since the "master" branch. For each TODO line, print
the file it's in, its line number, and the line, à la grep.

In order to include the 'bin.pm' file, we need to add its containing
folder to the include path. Do this by adding a Makefile with the
required command line argument.

Add a new test that modifies the test file to add a new new TODO line.
Test that it comes back in the `git todo` output.