dim: basic Fixes: check

Somewhat inspired by Stephen Rotwell's code, but because random
unfriendly encounters with bash it ended up looking quite a bit
different. Also only really checks sha1 for now, but that should be
good enough.

v2: Actually forward the error code from the subshell (which was
automatically created because of the pipeline, now I made it
explicit). Spotted by shellcheck, somehow I didn't notice that my
checks didn't actually abort the pull in my testing.

v3:
- quiet some noise in the output
- also check sha1 length and whether the subject matches. That latter
  check is a bit more strict than what sfr uses, but matches what dim
  cite produces.

Cc: Dave Airlie <airlied@linux.ie>
Acked-by: Dave Airlie <airlied@linux.ie>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>