grep: remove useless while condition stable

grep: remove useless while condition

A revised version of 35ba170c0f82 was sent to the list that fixed this
<http://markmail.org/message/jmfuiise5igcyh2m>, but the older version of
the patch was applied.

--- a/mercurial/commands.py	Mon Nov 12 19:27:03 2012 +0200
+++ b/mercurial/commands.py	Thu Nov 15 11:27:30 2012 -0600
@@ -2935,7 +2935,7 @@
     def matchlines(body):
         begin = 0
         linenum = 0
-        while True and begin < len(body):
+        while begin < len(body):
             match = regexp.search(body, begin)
             if not match:
                 break