bundle doc: replace "basis" with "prerequsite(s)"

bundle doc: replace "basis" with "prerequsite(s)"

In the preceding commits we introduced new documentation that talks
about "[commit|object] prerequsite(s)", but also faithfully moved
around existing documentation that talks about the "basis".

Let's change both that moved-around documentation and other existing
documentation in the file to consistently use "[commit|object]"
prerequisite(s)" instead of talking about "basis". The mention of
"basis" isn't wrong, but readers will be helped by us using only one
term throughout the document for this concept.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>