Validate with Git Hooks before accepting a merge request

We made possible to add some Git Hook based validation to things like commit message or commit author's email.

When we accept a merge request, it actually goes by the same validations an reject the merge if the merge commit does not follow Git Hook's predefined validations.

The problem is that it executes these validations when doing the very hard work of manipulating git repository, and cannot throw a usable and readable error to the user.

We are now executing these validations before scheduling git hard-work operations, and saving the error to display for the end-user.

Fixes #468 (closed), #305 (closed)