dim: ignore rr-cache when checking for dirty drm-rerere branch

Since we still have git gc fighting with our own gc algo it can happen
that git removes cached rerere entries before we decide to do. This is
mostly the case for unresolved conflicts (since there git deletes them
after 15 days already, while we keep them for 60 - the default only
for resolved conflicts).

To avoid confusion simply ignore this case. pull_rerere_cache should
clean up the mess if necessary.

v2: Fix typo while at it (Archit)

Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Reported-by: Archit Taneja <architt@codeaurora.org>
Cc: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>