ieot: default to not writing IEOT section

ieot: default to not writing IEOT section

As with EOIE, popular versions of Git do not support the new IEOT
extension yet.  When accessing a Git repository written by a more
modern version of Git, they correctly ignore the unrecognized section,
but in the process they loudly warn

	ignoring IEOT extension

resulting in confusion for users.  Introduce the index extension more
gently by not writing it yet in this first version with support for
it.  Soon, once sufficiently many users are running a modern version
of Git, we can flip the default so users benefit from this index
extension by default.

Introduce a '[index] recordOffsetTable' configuration variable to
control whether the new index extension is written.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>