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'

diff -r 3903775176ed b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123
diff -r 3903775176ed b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123