topics: add a new flag --age which will show last touched time for topics
This adds a new flag `--age` to `hg topic` command which will show topics
sorted by their last touched time and will also show the last touched time for
them.
This patch also adds a simple test to make sure the flag does not breaks by
future changes. Adding more tests showing output like "3 hours ago", "2 minutes
ago" etc will change as the code takes time.time() into account which will
increase with time, and hence the output will change, so we need some static
output like a date.