Fix IDE file dropdown

Closes #60021

See merge request gitlab-org/gitlab-ce!27015

**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`

**Why?**

Previously it managed this as an internal data property, but there
was extra complexity involved because the component doesn't have
all the information it needs (i.e. the `mouseOver` prop).

This is a good opportunity to colocate the data prop with the component
that has the information to change it (i.e. `file_row`) and let
`new_dropdown/index` receive this value as a prop.

**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

Resolves https://gitlab.com/gitlab-org/gitlab-ce/issues/57540

**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