doc: GIT_DEFAULT_HASH is and will be ignored during "clone"

doc: GIT_DEFAULT_HASH is and will be ignored during "clone"

The phrasing "is currently ignored" was prone to be misinterpreted
as if we were wishing if it were honored.  Rephrase it to make it
clear that the experimental variable will be ignored.

In the longer term, after/when we allow incremental/over-the-wire
migration of the object-format, i.e. cloning from an SHA-1
repository to create an SHA-256 repository (or vice versa) and
fetching and pushing between them would bidirectionally convert the
object format on the fly, it is likely that we would teach a new
option "--object-format" to "git clone" to say "you would use
whatever object format the origin uses by default, but this time, I
am telling you to use this format on our side, doing on-the-fly
object format conversion as needed".  So it is perfectly OK to
ignore the settings of this experimental variable, even after such
an extension happens that makes it necessary for us to have a way to
create a new repository that uses different object format from the
origin repository.

Signed-off-by: Junio C Hamano <gitster@pobox.com>