add more information to push event messages

[classabbyamp]

Adds number of commits, short hashes of before and after commits, and whether it was a force-push.

Example output:

classabbyamp pushed 5 commits to test (db6b6029 -> 9557e5ed, new HEAD: add something)
classabbyamp pushed 1 commit to test (9557e5ed -> f09714d3, new HEAD: change something)
classabbyamp force-pushed 5 commits to test (fe737118 -> cad7271e, new HEAD: add something)
classabbyamp force-pushed 1 commit to test (f09714d3 -> 2d47b36f, new HEAD: change something)