pull: configure --rebase via branch.<name>.rebase or pull.rebase

pull: configure --rebase via branch.<name>.rebase or pull.rebase

Since cd67e4d (Teach 'git pull' about --rebase, 2007-11-28),
fetch+rebase could be set by default by defining the config variable
branch.<name>.rebase. This setting can be overriden on the command line
by --rebase and --no-rebase.

Since 6b37dff (pull: introduce a pull.rebase option to enable --rebase,
2011-11-06), git-pull --rebase can also be configured via the
pull.rebase configuration option.

Re-implement support for these two configuration settings by introducing
config_get_rebase() which is called before parse_options() to set the
default value of opt_rebase.

Helped-by: Stefan Beller <sbeller@google.com>
Helped-by: Duy Nguyen <pclouds@gmail.com>
Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>