contrib/subtree: capture additional test-cases

contrib/subtree: capture additional test-cases

Patch series e7b07376e5 (Merge branch 'rs/subtree-fixes',
2018-10-26) corrects several defects in `git subtree split`.
The defects affect `split --rejoin` and merge commit processing.

There is no test coverage for this, and e7b07376e5 did not
introduce any.

Convert the minimum working example [1] from the original patch
submission [2] into test cases.

[1]: https://gist.github.com/FoxFireX/1b794384612b7fd5e7cd157cff96269e

[2]: <20180928183540.48968-1-roger.strain@swri.org>

Signed-off-by: Colin Stagner <ask+git@howdoi.land>
Signed-off-by: Junio C Hamano <gitster@pobox.com>