Add prolog predicate to determine number of parents
Add prolog predicate to determine number of parents

This change introduces a new prolog predicate for the
number of parents of a given patch set. This can be used to determine
from within the "rules.pl" file whether a change
is a merge commit or not.

This can be useful in order to evaluate a different set of
submit rules for merge commits.

Example usage:
submit_rule(submit(Parents)) :-
    Parents = label('Is-Merge', need(_)).
submit_rule(submit(Parents)) :-
    gerrit:commit_parent_count(A),
    A > 1,
    Parents = label('Is-Merge', ok(user(ID))).

Change-Id: I306163510e201606e936d95caea5ffa0bb957236

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