Make "rev-tree" able to read its own output again from the cache.

Also, add "date" information to the output so that you can do something
like this:

rev-tree `cat .git/HEAD` | sort -nr | cut -d' ' -f2 | while read i; do cat-file commit $i; done

which basically becomes a "git log" (aka "git changes") where things are
sorted by time.