Speed up features/notes_on_merge_requests_spec

This spec featured the slowest tests in the entire suite. After some
debugging, the cause was found to be the large commit diff generated by
comparing the stable and master branches.

To fix this, the seed repository was modified to create a simple branch
off of master that consists of three simple commits and minor changes.
The spec was then updated to compare master to this branch instead of
stable. The result is a spec group that runs in under 30 seconds, down
from about 90.
Speed up finders/merge_requests_finder_spec

Uses the :simple merge request factory trait introduced by d166e70; cuts
execution time of this spec in half.