Allow tracking branches to set up rebase by default.

Allow tracking branches to set up rebase by default.

Change cd67e4d4 introduced a new configuration parameter that told
pull to automatically perform a rebase instead of a merge.  This
change provides a configuration option to enable this feature
automatically when creating a new branch.

If the variable branch.autosetuprebase applies for a branch that's
being created, that branch will have branch.<name>.rebase set to true.

Signed-off-by: Dustin Sallings <dustin@spy.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>