Return the version control state of FILE.
A return of nil from this function means we have no information on the
status of this file. Otherwise, the value returned is one of:
'up-to-date The working file is unmodified with respect to the
latest version on the current branch, and not locked.
'edited The working file has been edited by the user. If
locking is used for the file, this state means that
the current version is locked by the calling user.
This status should *not* be reported for files
which have a changed mtime but the same content
as the repo copy.
USER The current version of the working file is locked by
some other USER (a string).
'needs-update The file has not been edited by the user, but there is
a more recent version on the current branch stored
in the repository.
'needs-merge The file has been edited by the user, and there is also
a more recent version on the current branch stored in
the repository. This state can only occur if locking
is not used for the file.
'unlocked-changes The working version of the file is not locked,
but the working file has been changed with respect
to that version. This state can only occur for files
with locking; it represents an erroneous condition that
should be resolved by the user (vc-next-action will
prompt the user to do it).
'added Scheduled to go into the repository on the next commit.
Often represented by vc-working-revision = "0" in VCSes
with monotonic IDs like Subversion and Mercurial.
'removed Scheduled to be deleted from the repository on next commit.
'conflict The file contains conflicts as the result of a merge.
For now the conflicts are text conflicts. In the
future this might be extended to deal with metadata
conflicts too.
'missing The file is not present in the file system, but the VC
system still tracks it.
'ignored The file showed up in a dir-status listing with a flag
indicating the version-control system is ignoring it,
Note: This property is not set reliably (some VCSes
don't have useful directory-status commands) so assume
that any file with vc-state nil might be ignorable
without VC knowing it.
'unregistered The file is not under version control.