Fix %timeit for slow functions, by Prabhu Ramachandran.

From his submission on the list:

Lets say you have a function that takes 2 seconds to evaluate.  With the
original code you are going to run that function 10 times regardless of the
fact that it already takes 2 seconds and with the original code, timing this
will take at least 60 seconds (with repeat=3) which is way too much.

IPython/Magic.py

@@ -1865,9 +1865,9 b' Currently the magic system has the following functions:\\n"""'
             # determine number so that 0.2 <= total time < 2.0
             number = 1
             for i in range(1, 10):
-                number *= 10
                 if timer.timeit(number) >= 0.2:
                     break
+                number *= 10
         
         best = min(timer.repeat(repeat, number)) / number