With this quick action the user can create a new MR starting from
the current issue using as `source_branch` the given `branch name` and
as `target_branch` the project default branch. If the `branch name` is
omitted a name is automatically created starting from the issue title.
In CE, this does nothing - the `MergeRequests::BuildService` will, at the time
of writing, never return a description for this case.
In EE, a project can have a default MR template, which will be returned by the
service. Previously we were only using the description passed in the params,
ignoring any already set on the object. Now we fall back to the one set on the
object if there was none in the params, allowing quick actions to be executed
from default MR templates when creating an MR from an issue.
Jacopo