cat: disable optimization of single file case for workingctx

It crashes because workingctx has no manifest.

mercurial/cmdutil.py

@@ -2403,7 +2403,7 b' def cat(ui, repo, ctx, matcher, prefix, '
         file = matcher.files()[0]
         mf = repo.manifest
         mfnode = ctx.manifestnode()
-        if mf.find(mfnode, file)[0]:
+        if mfnode and mf.find(mfnode, file)[0]:
             write(file)
             return 0
 

tests/test-cat.t

@@ -63,3 +63,8 b' Test fileset'
   tmp/h_45116003780e
   tmp/r_2
 
+Test working directory
+
+  $ echo b-wdir > b
+  $ hg cat -r 'wdir()' b
+  b-wdir