subtree: t7900: comment subtree_test_create_repo

It's unclear what the purpose of t7900-subtree.sh's `subtree_test_create_repo` helper function is. It wraps test-lib.sh's, `test_create_repo` but follows that up by setting log.date=relative. Why does it set log.date=relative? My first guess was that at one point the tests required that, but no longer do, and that the function is now vestigial. I even wrote a patch to get rid of it and was moments away from `git send-email`ing it. However, by chance when looking for something else in the history, I discovered the true reason, from e7aac44ed2 (contrib/subtree: ignore log.date configuration, 2015-07-21). It's testing that setting log.date=relative doesn't break `git subtree`, as at one point in the past that did break `git subtree`. So, add a comment about this, to avoid future such confusion. And while at it, go ahead and (1) touch up the function to avoid a pointless subshell and (2) update the one test that didn't use it. Signed-off-by: Luke Shumaker <lukeshu@datawire.io> Signed-off-by: Junio C Hamano <gitster@pobox.com>