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>

dim

@@ -2410,7 +2410,7 @@ function dim_cat_to_fixup # [branch]
 	repo=$(branch_to_repo $branch)
 	fixup_file=$(find_fixup_file $repo $branch)
 
-	cat > $fixup_file
+	cat >> $fixup_file
 	echo "Applied fixup for $branch"
 }