#C 162 bytes golfed:

main(int argc,char **argv){char *c=argv[1];int u=atoi(c),d;do{while(c[1]!=0){*c=abs(*c-*(c+1))+48;c++;}*c=0;c=argv[1];d=atoi(c);u+=d;}while(d>9);printf("%d",u);}

ungolfed:

main(int argc, char **argv)
{
    char *c=argv[1];
    int u=atoi(c),d;

    do
    {
        while(c[1]!=0)
        {
            *c=abs(*c-*(c+1))+48;
            c++;
        }

        *c=0;
        c=argv[1];
        d=atoi(c);
        u+=d;
    }
    while(d>9);

    printf("%d\n",u);
}

C 162 bytes

golfed:

main(int argc,char **argv){char *c=argv[1];int u=atoi(c),d;do{while(c[1]!=0){*c=abs(*c-*(c+1))+48;c++;}*c=0;c=argv[1];d=atoi(c);u+=d;}while(d>9);printf("%d",u);}

ungolfed:

main(int argc, char **argv)
{
    char *c=argv[1];
    int u=atoi(c),d;

    do
    {
        while(c[1]!=0)
        {
            *c=abs(*c-*(c+1))+48;
            c++;
        }

        *c=0;
        c=argv[1];
        d=atoi(c);
        u+=d;
    }
    while(d>9);

    printf("%d\n",u);
}