subrepo: add partial diff support for git subrepos
So far, git subrepositories were silently ignored for diffs.
This patch adds support for git subrepositories,
with the remark that --include and --exclude are not supported.
If --include or --exclude are used, the subrepo is ignored.
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 ..