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'

adding foo
adding bomb
adding a.c
adding dir/a.o
adding dir/b.o
! a.c
! dir/a.o
! dir/b.o
? .hgignore
a.c: unsupported file type (type is fifo)
! a.c