narrow: fix commits of empty files

The problem is that when committing a new file with empty contents (or
in general empty file with filelog p1 = -1), hg commit with narrow
doesn't create a filelog revision at all, which causes failures in
further commands.

The problem seems to be that:
- hg thinks that instead of creating a new filelog revision, it can
  use the filelog's p1 (the nullrev)
- because it thinks the file contents is the same in that revision and
  in p1
- because `narrowfilelog.cmp(nullrev, b'')` is True (unlike with
  `filelog.cmp`)

It's not clear to me which `cmp` behaves better. But I think it makes
sense to change the commit code to not to "reuse" the null rev when
adding an empty file with filelog p1 == filelog p2 == -1. This is
consistent with never writing the null rev in the manifest, which `hg
verify` claims is an invariant:

```
inside/c@4: manifest refers to unknown revision 000000000000
```

Differential Revision: https://phab.mercurial-scm.org/D11400

mercurial/commit.py

@@ -389,6 +389,7 b' def _filecommit('
     text = fctx.data()
     if (
         fparent2 != repo.nullid
+        or fparent1 == repo.nullid
         or meta
         or flog.cmp(fparent1, text)
         or force_new_node

tests/test-narrow-commit.t

@@ -105,14 +105,7 b" debugdirstate. If we don't do this, the "
   $ hg debugdirstate --no-dates
   n 644         10 set                 inside/f1
 
-Can't commit empty files
+Can commit empty files
 
   $ touch inside/c; hg add inside/c; hg commit -qm _; hg verify -q
-   warning: revlog 'data/inside/c.i' not in fncache!
-   4: empty or missing inside/c
-   inside/c@4: manifest refers to unknown revision 000000000000
-  1 warnings encountered!
-  hint: run "hg debugrebuildfncache" to recover from corrupt fncache
-  2 integrity errors encountered!
-  (first damaged changeset appears to be 4)
-  [1]
+  $ hg cat -r . inside/c