Simplify Mentionable concern instance methods

We remove some arguments that are rarely used or 
used just to simplify setups on specs.

Modified Mentionable#create_new_cross_references method 
we don’t need to calculate previous references to avoid the 
duplication because we do that at database level when 
creating references extracted from the current entity state.

MergeRequests won’t create cross_references for commits that are included so we change a spec to use a different merge request to make references to commits to other branches

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