stream: in v3, skip the "size" fast path if the entries as some unknown size
We are about to prefetch size during the lock less in the v3 case. So we need to
avoid trying to use that prefetched size when it is not available.
See next changeset for the motivation.
$ hg init repo
$ cd repo
$ cat > a <<EOF
> a
> b
> c
> EOF
$ hg ci -Am adda
adding a
$ cat > a <<EOF
> d
> e
> f
> EOF
$ hg ci -m moda
$ hg diff --reverse -r0 -r1
diff -r 2855cdcfcbb7 -r 8e1805a3cf6e a
--- a/a Thu Jan 01 00:00:00 1970 +0000
+++ b/a Thu Jan 01 00:00:00 1970 +0000
@@ -1,3 +1,3 @@
-d
-e
-f
+a
+b
+c
$ cat >> a <<EOF
> g
> h
> EOF
$ hg diff --reverse --nodates
diff -r 2855cdcfcbb7 a
--- a/a
+++ b/a
@@ -1,5 +1,3 @@
d
e
f
-g
-h
should show removed file 'a' as being added
$ hg revert a
$ hg rm a
$ hg diff --reverse --nodates a
diff -r 2855cdcfcbb7 a
--- /dev/null
+++ b/a
@@ -0,0 +1,3 @@
+d
+e
+f
should show added file 'b' as being removed
$ echo b >> b
$ hg add b
$ hg diff --reverse --nodates b
diff -r 2855cdcfcbb7 b
--- a/b
+++ /dev/null
@@ -1,1 +0,0 @@
-b