Fix git command for detecting changed files in multi-parent merge commits

[h4iku]

When the fixed commit is a merge commit with multiple parents, the checkout script's git show --name-only command fails to list changed files because Git cannot determine which parent to compare against. As a result, some fixed checkout versions remain in a buggy state with updated tests, causing those tests to fail.

An example is the bug 2 of luigi, where the fix commit is a merge commit with two parents and running git show --name-only on it returns only the commit message:

$ git show --name-only
commit 24e85945ae39e5975491527e00c3f0f64b42ea6e (HEAD)
Merge: baa54c9f 5d9d38df
Author: Honnix <honnix@users.noreply.github.com>
Date:   Thu May 9 13:23:23 2019 +0200

    Merge pull request #2705 from spotify/claire/fix_dataflow_bq_return_type

    Fix BigQueryTarget parsing in beam_dataflow module

Changing the command to git show --name-only --first-parent returns the expected two changed files:

$ git show --name-only --first-parent
commit 24e85945ae39e5975491527e00c3f0f64b42ea6e (HEAD)
Merge: baa54c9f 5d9d38df
Author: Honnix <honnix@users.noreply.github.com>
Date:   Thu May 9 13:23:23 2019 +0200

    Merge pull request #2705 from spotify/claire/fix_dataflow_bq_return_type

    Fix BigQueryTarget parsing in beam_dataflow module

luigi/contrib/beam_dataflow.py
test/contrib/beam_dataflow_test.py