SF Patch 681780: Faster commonprefix (OS independent)

Improved based on discussions at:

http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/252177
http://groups.google.com/groups?th=fc7b54f11af6b24e&seekm=bss2so$om$00$1@news.t-online.com

Lib/posixpath.py

@@ -123,16 +123,13 @@ def dirname(p):
 def commonprefix(m):
     "Given a list of pathnames, returns the longest common leading component"
     if not m: return ''
-    prefix = m[0]
-    for item in m:
-        for i in range(len(prefix)):
-            if prefix[:i+1] != item[:i+1]:
-                prefix = prefix[:i]
-                if i == 0:
-                    return ''
-                break
-    return prefix
-
+    s1 = min(m)
+    s2 = max(m)
+    n = min(len(s1), len(s2))
+    for i in xrange(n):
+        if s1[i] != s2[i]:
+            return s1[:i]
+    return s1[:n]
 
 # Get size, mtime, atime of files.