git: Make Commit.age and Branch.age properties.

Also, instead of passing today, use datetime.datetime.today() directly. The original way of passing today did not work properly. Indeed, when the function was called several times, today had the same value because datetime.datetime.today() was evaluated only once.
git: Add a new method to Commit: short_subject().
test_git: Refactor Commit tests.
test_git: Refactor Branch tests.
test_web: Remove unused local variables.
test_git: Wrap long lines.