A path commonPrefix function

[marks]

I have the following `commonPrefix()` function (for whole paths) which works 
fine on Linux (and Windows), but wondering if it could be shorter and/or more 
efficient?
    
    
    proc commonPrefix*(paths: openArray[string], sep = "/"): string =
      if len(paths) == 0:
        return ""
      result = paths[0]
      for path in paths[1 .. ^1]:
        while not path.startsWith(result) and len(result) > 0:
          result = result[0 .. ^2]
      result = result[0 .. result.rfind(sep)]
    
    
    Run

Tests: 
    
    
    test "commonPrefix":
      check(commonPrefix(["/tmp/test1", "/tmp/data.dat"]) == "/tmp/")
      check(commonPrefix(["/home/mark/test1", "/tmp/data.dat"]) == "/")
      check(commonPrefix(["/home/mark/test1", "local/data.dat"]) == "")
    
    
    Run