dim: symbolic link for the rr-cache

After about a month and a few attempts it's clear that my rr-cache gc
logic is busted. When copying stuff back&forth between the git branch
and git's rr-cache dir, something somewhere (or maybe it's git rerere
itself) touches all files and wreaks the timestamps.

End result is that over this month, every time when someone gc'ed a
few files, they've been resurrected a few days later by someone else.

I think the only way to fix this is by dropping the copying and
replacing git's rr-cache with a symbolic link. That way, when we
delete an rr-cache entry in the git branch, it won't be able to
resurrect. I hope at least ...

This also makes dim revert-rerere no longer needed, delete it.

v2: Unfunny the ln option placement (Jani).

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