Utils.hs: function to chomp prefix + separator from a string

Some auto-repair tags are composed of a prefix and then an optional
argument; the new "chompPrefix" function in Utils.hs allows to strip a
prefix, allowing the last character (the separator) to be absent if there
is no argument.

Signed-off-by: Dato Simó <dato@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>