Reproducing bugs without issues

[gavanderhoorn]

We tend to use the timestamp associated with the PR that introduces the commit that fixes whatever is described in the PR.

This works in most cases, as when the PR is merged it's not immediately released, so there is still a "last release was with the bug"-state to be found in ros/rosdistro.

However that is rather inexact and depends on "luck" almost: we cannot guarantee that the previously released version of a package actually contains the issue described in the bug.

I'm not asking for changes to the process we currently use here, rather I'd wanted to make this explicit and potentially gather some opinions.

[gavanderhoorn]

An example of such a bug is geometry2/001fca6.

In #65 I've generated a .rosinstall file for it, but as noted in #65 (comment) I've had to use the date & time of when the PR was created, as there is no issue.