In a subsequent commit, index-pack will be taught to write ".promisor"
files which are similar to the ".keep" files it knows how to write.
Refactor the writing of ".keep" files, so that the implementation of
writing ".promisor" files becomes easier.
Signed-off-by: Jonathan Tan <jonathantanmy@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>