strutil: use absolute_import

This file doesn't import anything. But chances are we'll eventually
implement a static analysis check that ensures absolute_import is used.
So we might as well add this.

mercurial/strutil.py

@@ -5,6 +5,8 b''
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
+from __future__ import absolute_import
+
 def findall(haystack, needle, start=0, end=None):
     if end is None:
         end = len(haystack)