tools/wrap-commit-msg.py: fix file truncation

truncate() operates from the current file pointer position. On at least
Linux specifying 0 without resetting the pointer will pad the file with
zeros to the current offset.

Change-Id: Ide704a1097f46c0c530f27212bb12e923f93e2d6

tools/wrap-commit-msg.py

@@ -59,6 +59,7 @@ def main(fileobj):
     if fileobj == sys.stdin:
         fileobj = sys.stdout
     else:
+        fileobj.seek(0)
         fileobj.truncate(0)
     fileobj.write(output)