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>

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