dirstate: use self._filename instead of immediate string `dirstate`
This prevents immediate string `dirstate` from multiplying. This is also
a preparation for making dirstate aware of PENDING mechanism in
subsequent patches.
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 set bar
n 644 -1 set foo
status
$ hg st -A
! bar
? baz
C foo
$ cd ..