dim: Update remote before updating for-linux-next branches

Noticed by Archit, but probably a good idea in general.

v2: Fetch before printing (Jani). While at it make them all less
noisy, it's kinda confusing to have random git output interleaved with
dim messages.

v3: Don't silence stderr (Jani)

v4: Use git fetch -q (Abdiel).

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>