Allow the rebase command to be defined

This is useful if one wants to use StGIT on top of a Subversion
repository. After a "git svn clone" and "stg init", the following
options have to be defined to allow "stg pull" to work properly:

branch.master.remote = svn
stgit.pull-policy = fetch-rebase
stgit.fetchcmd = git svn fetch
stgit.rebasecmd = git svn rebase

Signed-off-by: Catalin Marinas <catalin.marinas@gmail.com>