move __del__ from httprepository to basehttphandler
This should prevent the next patch from reopening an issue fixed by
0d94e4a3ddb4.
#!/bin/sh
hg init repo
cd repo
touch foo
hg ci -Am 'add foo'
hg up -C null
# this should be stored as a delta against rev 0
echo foo bar baz > foo
hg ci -Am 'add foo again'
hg debugindex .hg/store/data/foo.i