stablesortcache: update the variable tracking on-disk state after write
Since we updated the on disk content, we should update that value.
In practice the object will likely be discarded after the write, but there is
nothing wrong in being more correct.
#require test-repo pyflakes
Copied from Mercurial core (60ee2593a270)
$ cd "`dirname "$TESTDIR"`"
run pyflakes on all tracked files ending in .py or without a file ending
(skipping binary file random-seed)
$ hg locate 'set:(**.py or grep("^#!.*python")) - removed()' 2>/dev/null \
> | xargs pyflakes 2>/dev/null