Print the latest commit considered or timestamp

[consideRatio]

I run this with --since sha1234 and update a changelog as PRs come in. Right now for example, for JupyterHub's upcoming release.

After having done this some times I realize it would be nice to see output that allows me to later write a --since <sha> and having that sha printed out along with info such as...

repo:jupyterhub/jupyterhub type:pr


Found 104 items, which will take 3 pages

I realize that if you want to consider issues, the timestamp is more relevant than the latest commit. So I think we should simply print the current time in UTC or similar as compared to the latest commit.. Or both.

[choldgraf]

Sounds good to me - perhaps a timestamp for beginning and end of the window within the changelog somewhere would be good