fix an exception in gpg.py with multiples sigs for the same cset
- silly error, do not overwrite the variable we use for iterating
- correct 'hg help sign'
1:3aa14bbc23d90:8633637036c13aa14bbc23d9 tip8633637036c18633637036c1+8633637036c13aa14bbc23d9 tipmerging file13aa14bbc23d9 tip3aa14bbc23d9 tip3aa14bbc23d9 tip3aa14bbc23d9 tip