identify: simplify the dirty check

This is equivalent to the previous code, but it seems better to be explicit
about what aspects of dirty are being ignored.  Perhaps they shouldn't be, since
the help text says 'followed by a "+" if the working directory has uncommitted
changes'.  Both merges and branch changes are committable, even if the files are
unchanged.

Additionally, this will make the `identify` command notice missing subrepo
files, once subrepos are taught to look for missing files.

mercurial/commands.py

@@ -2775,8 +2775,7 b' def identify(ui, repo, source=None, rev='
                 taglist.extend(p.tags())
 
             dirty = ""
-            if (any(repo.status())
-                or any(ctx.sub(s).dirty() for s in ctx.substate)):
+            if ctx.dirty(missing=True, merge=False, branch=False):
                 dirty = '+'
             fm.data(dirty=dirty)