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.
workingfilectx.date = (1000, 0)
ASCII : Gr?ezi!
Latin-1 : Grüezi!
UTF-8 : Grüezi!