Add first-parent oneline log to the output of fast_forward()

Always append the merge log to the commit message in rmerge() even when a
top_message is passed
Parse fast-forward first-parent log and list merged PRs

Produce a digest of the merged PRs during the fast-forward using the
PullRequest __repr__ method. This prevents links to be created to the
corresponding issues when pushing the merge commit Github.
merge master -D org -I include -E exclude: PR 92 (Unrebased PRs improvement)
merge master -D org -I include -E exclude: PR 93 (Update submodules command: add first-parent log to the commit message)