Rewrite log command. New version is faster and more featureful. The original implementation of log walked backwards through history, which had terrible behaviour. It took several minutes to view complete kernel change history on a fast machine, for example. The rewrite uses a windowed approach to walk hunks of history forwards, while still giving results in reverse order. This reduces run time from five minutes to five seconds on my system. In addition, the rewrite uses our normal name handling mechanisms, so you can run a command like "hg log net/ipv4/**.c" and get a useful answer. It optimises for three different cases (no arguments, only files, and anything goes), so it performs well in all circumstances I've tested.