New `Api.projectMergeRequests` allows:
- to query for all MRs on a project without specifying `mergeRequestId`
- to filter the returned MRs using parameters from
https://bit.ly/2H7We4V
The new API request is used for fetching information about MRs
associated with a particular branch in Web IDE to have IdeSidebar
behave consistently in both scenarios:
- getting to a branch from and MR (where we already have info about
relevant MR), or
- getting to a branch somehow differently directly
For cases where there are several merge requests that the current branch
is associated with, mark the most recent one as 'current'
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/49663
