It looks to be a stub/mock rather than strictly an expectation of the
system, so dropping this to only a single invocation expected, as we've
removed one of the two places #last_diff_file would be invoked.

Before this fix, a suggestion which just removes
an empty line wasn't appliable

- added suggestions to mock data
- fixed props to be not required

Adds the groundwork needed in order to persist multi-line suggestions,
while providing the parsing strategy which will be reused for the
**Preview** as well.

This is an issue that can only be seen through EE. Further
details can be seen on
https://gitlab.com/gitlab-org/gitlab-ee/issues/9876. In general
we should always use the latest diff version of a file in order
to both create and apply suggestions.

1. Presents the system error message when available,
when applying suggestions

2. Adjusts target branch updates affecting MRs pointing
to it when there are previously created suggestions in
the MR. We just need to compare the HEAD shas from
position and MR source branch.

1. Avoid suggestions being applied on the same file
from reverting previous changes
2. Gracefully use and handle file changes error
when updating the file (though, it does not totally
solves the sync problem for multiple suggestion
applications at once)