extdiff: allow modifications in subrepos to be copied back

This check was a legacy bit from when the file data was being fetched manually
with 'ctx[wfn]', but archive() does that now.  49966b5ab16f seems to indicate
that this avoided a problem where a merge adds a file to another branch, and
that test still passes.

Unfortunately, I don't see a way to create a test that modifies the file in the
temporary directory before the command exits.

I wonder if the os.lstat() call needs to be wrapped in an exception handler for
the case where archive didn't create a file because the file didn't exist in
that revision.  But I wasn't able to trigger a problem without it on a real
repository.

hgext/extdiff.py

@@ -101,12 +101,8 b' def snapshot(ui, repo, files, node, tmpr'
                          matchfn=scmutil.matchfiles(repo, files),
                          subrepos=listsubrepos)
 
-        ctx = repo[node]
         for fn in sorted(files):
             wfn = util.pconvert(fn)
-            if wfn not in ctx:
-                # File doesn't exist; could be a bogus modify
-                continue
             ui.note('  %s\n' % wfn)
 
             if node is None: