commit-msg hooks complains on merge commits

I encountered this when fixing conflicts in a develop-olympus -> develop merge and committing with git ci.

Warning: heading should be a sentence beginning with a capital letter.
You entered:
Merge branch 'develop-olympus' into develop
Syntax error on line 1:
>> Merge branch 'develop-olympus' into develop
Expecting: comment, header,
[develop 7d6b289] Merge branch 'develop-olympus' into develop

We should detect if a commit being made is a merge commit (>1 parent?) and skip the checks in that case.