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.

For a good visual, [this image](https://s3.amazonaws.com/f.cl.ly/items/3P1D2U3L301g2N1b3U1o/screenshot.png) is what was being rendered on every single
spec, and [this](https://s3.amazonaws.com/f.cl.ly/items/2H0s2F3c1J0D0J1o0M2F/screenshot2.png) is what it is now.