pull: introduce a pull.rebase option to enable --rebase

pull: introduce a pull.rebase option to enable --rebase

Currently we either need to set branch.<name>.rebase for existing
branches if we'd like "git pull" to mean "git pull --rebase", or have
the forethought of setting "branch.autosetuprebase" before we create
the branch.

Introduce a "pull.rebase" option to globally configure "git pull" to
mean "git pull --rebase" for any branch.

This option will be considered at a lower priority than
branch.<name>.rebase, i.e. we could set pull.rebase=true and
branch.<name>.rebase=false and the latter configuration option would
win.

Reviewed-by: Sverre Rabbelier <srabbelier@gmail.com>
Reviewed-by: Fernando Vezzosi <buccia@repnz.net>
Reviewed-by: Eric Herman <eric@freesa.org>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Liked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>