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.

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.

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).

Print usage information with `-h`.

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.