tests/test-rebuildstate.t

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.

basic test for hg debugrebuildstate

  $ hg init repo
  $ cd repo

  $ touch foo bar
  $ hg ci -Am 'add foo bar'
  adding bar
  adding foo

  $ touch baz
  $ hg add baz
  $ hg rm bar

  $ hg debugrebuildstate

state dump after

  $ hg debugstate --nodates | sort
  n 644         -1 bar
  n 644         -1 foo

status

  $ hg st -A
  ! bar
  ? baz
  C foo