parsers: fix a memleak, and add a clearcaches method to the index
This change also fixes a nasty memory leak: previously, self->caches
was not being freed.
The new clearcaches method lets us benchmark with finer granularity,
as it lets us separate the cost of loading a revlog index from those
of populating and accessing the cache data structures.
Issue351: mq: qrefresh can create extra revlog entry
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ hg qinit
$ echo b > b
$ hg ci -A -m foo
adding b
$ echo cc > b
$ hg qnew -f foo.diff
$ echo b > b
$ hg qrefresh
$ hg debugindex b
rev offset length base linkrev nodeid p1 p2
0 0 3 0 0 1e88685f5dde 000000000000 000000000000