scmutil: split platform-specific bits into their own modules
This parallels what's done for the util module, which imports either
mercurial.posix or mercurial.windows as 'platform' and then slurps the
appropriate functions into its own namespace.
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
$ cd ..