magit-request-pull: new command

Close #1149.
magit-rebase{,-onto}: use magit-rebase-async
magit-rebase: actually use the optional ARGS
magit-section-region-siblings: preserve order
magit-am-popup: sanitize arguments
prepare log margin for reflogs too
magit-anything-unmerged-p: new function
magit-visit-file: start smerge session when appropriate
magit-discard-files: present files in original order