Since -fixes and -fixes-next (to a lesser extent) are rebasing trees in
drm-misc, add a dim rebase command that sanity checks the upstream and
adds SoB for the committer.

Changes in v2:
- s/validate_upstream_branch/validate_upstream_baseline/ (Daniel)
- Use check_conflicts instead of hand rolling (Daniel)

Cc: Boris Brezillon <boris.brezillon@bootlin.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180828183100.189905-1-sean@poorly.run