List git commits in reverse order.
When posting a branch with many commits, it's likely that the first one is the

most important one (with the best description), and the later ones are fixups.

This change alters extract_summary and extract_description to reflect that.

This also brings us into line with how the hg client works.

Created a branch with several commits and posted it. Saw that the summary was

that of the first commit, and that the commits were listed in chronological

rather than reverse chronological order.