Fix -n bug; default to all ancestors

When given no arguments, log.c would output nothing. This differs from `git log`'s defaults. `log.c` is an attractive place for newbies like myself to get started, and it's confusing trying to work out why it doesn't seem to be working. This PR gives it the same default behaviour as `git log`, i.e. to list all ancestors.

Separately, `log -n 1` had this bug:
```
$ log -n 1
Argument -n not followed by valid count: -n
usage: log [<options>]
```