(🎁) Support relative `base-dir`

[KotlinIsland]

So basically I was trying to invoke mypy-primer with --base-dir ./workdir but it failed, saying that it couldn't find mypy. I think it was looking for workdir/new_mypy/venv/bin/mypy, but the cwd is ./workdir/projects/mypy.

👉 mypy_primer --repo ../mypy --old HEAD -k mypy --base-dir ./workdir

mypy
https://github.com/python/mypy
----------

old mypy
> workdir/old_mypy/venv/bin/mypy --config-file mypy_self_check.ini -p mypy -p mypyc --no-incremental --cache-dir=/dev/null --show-traceback --soft-error-limit ' -1'
UNEXPECTED FAILURE
        /bin/sh: 1: workdir/old_mypy/venv/bin/mypy: not found
----------

new mypy
> workdir/new_mypy/venv/bin/mypy --config-file mypy_self_check.ini -p mypy -p mypyc --no-incremental --cache-dir=/dev/null --show-traceback --soft-error-limit ' -1'
UNEXPECTED FAILURE
        /bin/sh: 1: workdir/new_mypy/venv/bin/mypy: not found
----------

diff
        - /bin/sh: 1: workdir/old_mypy/venv/bin/mypy: not found
        + /bin/sh: 1: workdir/new_mypy/venv/bin/mypy: not found
==========

The solution was to invoke it with --base-dir (pwd)/workdir(🐟), but this is not straightforward from the output displayed, I think it would be good if this usage was supported.

The change is simply just absolute-ifying the base-dir, I'm okay with this, but perhaps there's thing's I'm not accounting for. I'm happy to rework this if there are any alternative suggestions.

[hauntsaninja]

Thanks!