repeat_string() - repeat a string a certain number of times.

string repeat_string(string str, int repeats);

returns a new string

- repeats <= 0: ""
- repeats > 0 original string repeated either int times or the maximum
  number of times without getting greater than the maximum string length

maxstrlen = 5:
repeat_string("ab", 3) = "abab"

maxstrlen >=6:
repeat_string("ab", 3) = "ababab"