largefiles: optimize status by synchronizing lfdirstate with the largefile on update

This speeds up status on a largefiles repo by synchronizing the largefiles dirstate to the
largefile's mtime upon update, preventing the files from coming back as "unsure" later,
requiring a check of the SHA1 sum.
localrepo: add "vfs" fields to "localrepository" for migration from "opener"

As a part of migration to vfs, this patch adds "vfs" fields to
"localrepository" class.

This allows new codes to access current "opener" objects related to
repositories via "vfs" fields, so patches referring to "vfs" will
replace referring to "opener" in time.

This patch also adds initializations for "vfs" fields to
"statichttprepository" class derived from it, because its constructor
doesn't invoke the constructor of "localrepository", so "vfs" fields
should be initialized explicitly as same as "opener" fields: it has no
working directory, so "wvfs" field is not added.
localrepo: use path expansion API via vfs

As a part of migration to vfs, this patch moves path expansion API
invocations in the constructor of "localrepository" to the constructor
of "opener", because the root path to the repository is very important
to handle paths using non-ASCII characters correctly.

This patch also rearrange initialization order of "wvfs" field,
because it is required to initialize "self.root".
localrepo: use "self.wvfs.join()" instead of "os.path.join()"

As a part of migration to vfs, this patch uses "self.wvfs.join()"
instead of "os.path.join()", while initialization of fields in the
constructor of "localrepository" class.
localrepo: use the path relative to "self.vfs" instead of "path" argument

As a part of migration to vfs, this patch uses "self.root", which can
be recognized as the path relative to "self.vfs", instead of "path"
argument.

This fix allows to make invocations of "util.makedirs()" and
"os.path.exists()" while ensuring repository directory in
"localrepository.__init__()" ones indirectly via vfs.

But this fix also raises issue 2528: "hg clone" with empty destination.

"path" argument is empty in many cases, so this issue can't be fixed
in the view of "localrepository.__init__()".

Before this patch, it is fixed by empty-ness check ("not name") of
exception handler in "util.makedirs()".

    try:
        os.mkdir(name)
    except OSError, err:
        if err.errno == errno.EEXIST:
            return
        if err.errno != errno.ENOENT or not name:
            raise

This requires "localrepository.__init__()" to invoke "util.makedirs()"
with "path" instead of "self.root", because empty "path" is treated as
"current directory" and "self.root" becomes valid path.

But "hg clone" with empty destination can be detected also in
"hg.clone()" before "localrepository.__init__()" invocation, so this
patch re-fixes issue2528 by checking it in "hg.clone()".
localrepo: use "vfs" intead of "opener" while ensuring repository directory

As a part of migration to vfs, this patch uses "self.vfs" instead of
"self.opener", while ensuring repository directory in the constructor
of "localrepository" class.
localrepo: use file API via vfs while ensuring repository directory

As a part of migration to vfs, this patch invokes some file API
indirectly via vfs, while ensuring repository directory in the
constructor of "localrepository" class.

New file API are added to "scmutil.abstractopener" class, because they
are also used via other derived classes than "scmutil.opener".

But "join()" is not yet defined other than "scmutil.opener" class,
because it should not be used via other opener classes yet.