dim: store Link: in MERGE_MSG in apply-pull

This way there's no need for a dim apply-pull-continue, plain old git
commit is enough.

Aside: We might want to do the same trick for dim apply-branch, but
git am is a bit harder to script.

v2: Drop cat, use shell redirects to appeas shellcheck.

Acked-by: Jani Nikula <jani.nikula@intel.com>
Cc: Dave Airlie <airlied@gmail.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>