Teach 'git pull' about --rebase

Teach 'git pull' about --rebase

When calling 'git pull' with the '--rebase' option, it performs a
fetch + rebase instead of a fetch + merge.

This behavior is more desirable than fetch + pull when a topic branch
is ready to be submitted and needs to be update.

fetch + rebase might also be considered a better workflow with shared
repositories in any case, or for contributors to a centrally managed
repository, such as WINE's.

As a convenience, you can set the default behavior for a branch by
defining the config variable branch.<name>.rebase, which is
interpreted as a bool.  This setting can be overridden on the command
line by --rebase and --no-rebase.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>