largefiles: make cat on standins do something

cat of a standin would silently fail.

The use of standins is mostly an implementation detail, but it is already a bit
leaking. Being able to see the content of standins might be convenient for
debugging.

hgext/largefiles/overrides.py

@@ -1116,9 +1116,11 b' def overridecat(orig, ui, repo, file1, *'
     m = scmutil.match(ctx, (file1,) + pats, opts)
     origmatchfn = m.matchfn
     def lfmatchfn(f):
+        if origmatchfn(f):
+            return True
         lf = lfutil.splitstandin(f)
         if lf is None:
-            return origmatchfn(f)
+            return False
         notbad.add(lf)
         return origmatchfn(lf)
     m.matchfn = lfmatchfn
@@ -1131,7 +1133,7 b' def overridecat(orig, ui, repo, file1, *'
         fp = cmdutil.makefileobj(repo, opts.get('output'), ctx.node(),
                                  pathname=f)
         lf = lfutil.splitstandin(f)
-        if lf is None:
+        if lf is None or origmatchfn(f):
             # duplicating unreachable code from commands.cat
             data = ctx[f].data()
             if opts.get('decode'):

tests/test-largefiles.t

@@ -1506,6 +1506,12 b' Cat a largefile'
   large4-modified
   $ hg --cwd sub cat -r '.^' ../normal3
   normal3-modified
+Cat a standin
+  $ hg cat .hglf/sub/large4
+  e166e74c7303192238d60af5a9c4ce9bef0b7928
+  $ hg cat .hglf/normal3
+  .hglf/normal3: no such file in rev 598410d3eb9a
+  [1]
 
 Test that renaming a largefile results in correct output for status