Previously, the file paths of TODO lines in the output were relative to
the repository root. When you're in a subdirectory, though, this makes
them harder to work with, as they're relative paths, but aren't relative
to the current working directory.

Change the file path output so that paths are relative to the current
directory instead of the repo root.

I wasn't clearning up the "subdir" directory created in this test. Tried
adding a new test that used the same subdirectory name, which failed to
create the directory because it was already created here.

Add a couple tests to confirm that the program works for files that have
been modified:

* in the working directory but not staged
* in the index but not committed

Previously, untracked files were not included in the diff, so TODO lines
in those files would not be printed by the program.

Add the untracked `DiffOptions` to include the content of these files
when searching for TODOs.

Previously, running `git todo` outside the root of the repository would
result in this error:

    error: unable to open repository: could not find repository from '.'; class=Repository (6); code=NotFound (-3)

It turns out the `open()` method expects the given path to be the
repository root. Use `open_from_env()` instead, which functions more
like regular Git commands.

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.