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.
Add merge message to the body of the submodules update PR
Restore generated by build footer in the PR body
Add integration test for update-submodule PR
Merge pull request #93 from sbesson/submodule_log

Update submodules command: add first-parent log to the commit message