Rename submodule.<name>.rebase to submodule.<name>.update

Rename submodule.<name>.rebase to submodule.<name>.update

The addition of "submodule.<name>.rebase" demonstrates the usefulness of
alternatives to the default behaviour of "git submodule update". However,
by naming the config variable "submodule.<name>.rebase", and making it a
boolean choice, we are artificially constraining future git versions that
may want to add _more_ alternatives than just "rebase".

Therefore, while "submodule.<name>.rebase" is not yet in a stable git
release, future-proof it, by changing it from

  submodule.<name>.rebase = true/false

to

  submodule.<name>.update = rebase/checkout

where "checkout" specifies the default behaviour of "git submodule update"
(checking out the new commit to a detached HEAD), and "rebase" specifies
the --rebase behaviour (where the current local branch in the submodule is
rebase onto the new commit). Thus .update == checkout is equivalent to
.rebase == false, and .update == rebase is equivalent to .rebase == true.
Finally, leaving .update unset is equivalent to leaving .rebase unset.

In future git versions, other alternatives to "git submodule update"
behaviour can be included by adding them to the list of allowable values
for the submodule.<name>.update variable.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>