obslog: redefine default output as a template, unless -f was given This was easier than I expected to do without affecting any test output at all. `hg obslog -f` is pretty broken with templating. For example, it inserts a single "date" field, but it provides two values for it if the dates are different. That results in a traceback when running e.g. `hg obslog -f -Tfoo` with non-local obsmarkers with different dates. We need to figure out what we want to put in the template for the filtered case. I've just left it out for now.