**Why?**
Currently the IDE loads a merge request based on only the
`source_branch` name. This means it loads MR's from
forks that have the same branch name (not good).

- This required updating the BE API to accept `source_project_id`

**What?**

A Vue warning that `ide_status_bar` sent a `Boolean` to a `String`
property (`img-src).

**What was the fix?**

Previously, `latestPipeline` could be one of the following values:

|          |        |
|----------|--------|
| `null`   | The pipeline hasn't loaded yet |
| `false`  | The pipeline has loaded, but nothing was returned. |
| `Object` | The piepline has loaded. |

Giving a semantic meaning to different falsey values hurts
maintainability. This commit fixes the above problem by removing the
`false` value and introducing a `hasLoadedPipeline` state property.

Upgrade to latest Version

- fixed getter spec with correct getter name

**How?**
Previously the files_decorator inserted parent folders inefficiently.
It started at the first part and ensured each was inserted.

Since the file entries are given to use in alphabetical order, we can
start at the last part of the blob's parents. If this exists, we can
short circuit and be done inserting parents.

**What else?**
- Improve performance of decorateData. The object spread operator is
not needed because the object is brand new.

Link to the MR from current branch in Web IDE

Closes #49663

See merge request gitlab-org/gitlab-ce!25689

 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

- fixed test spec
- added warning only if file is deleted