dim: Append to fixup file

If we already have a fixup file for a certain branch, we would override
any previous fixups. Just appending to the previous diff (even if
already containing the same path) seems to work better.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>