bundle2: lazily iterate over bundle parts in the test

We used to create a list to know the number of parts in the bundle. This prevents
any lazy reading as planned for real usage.

The list creation is dropped. Some test output changed as debug output is
now interleaved with command output.

tests/test-bundle2.t

@@ -146,13 +146,14 b' Create an extension to test bundle2 API'
   >         value = params[key]
   >         if value is not None:
   >             ui.write('    %s\n' % value)
-  >     parts = list(unbundler)
-  >     ui.write('parts count:   %i\n' % len(parts))
-  >     for p in parts:
+  >     count = 0
+  >     for p in unbundler:
+  >         count += 1
   >         ui.write('  :%s:\n' % p.type)
   >         ui.write('    mandatory: %i\n' % len(p.mandatoryparams))
   >         ui.write('    advisory: %i\n' % len(p.advisoryparams))
   >         ui.write('    payload: %i bytes\n' % len(p.data))
+  >     ui.write('parts count:   %i\n' % count)
   > EOF
   $ cat >> $HGRCPATH << EOF
   > [extensions]
@@ -346,7 +347,6 b' Test part'
 
   $ hg statbundle2 < ../parts.hg2
   options count: 0
-  parts count:   5
     :test:empty:
       mandatory: 0
       advisory: 0
@@ -367,6 +367,7 b' Test part'
       mandatory: 0
       advisory: 0
       payload: 0 bytes
+  parts count:   5
 
   $ hg statbundle2 --debug < ../parts.hg2
   start processing of HG20 stream
@@ -378,51 +379,51 b' Test part'
   part id: "0"
   part parameters: 0
   payload chunk size: 0
+    :test:empty:
+      mandatory: 0
+      advisory: 0
+      payload: 0 bytes
   part header size: 17
   part type: "test:empty"
   part id: "1"
   part parameters: 0
   payload chunk size: 0
+    :test:empty:
+      mandatory: 0
+      advisory: 0
+      payload: 0 bytes
   part header size: 16
   part type: "test:song"
   part id: "2"
   part parameters: 0
   payload chunk size: 178
   payload chunk size: 0
+    :test:song:
+      mandatory: 0
+      advisory: 0
+      payload: 178 bytes
   part header size: 43
   part type: "test:math"
   part id: "3"
   part parameters: 3
   payload chunk size: 2
   payload chunk size: 0
+    :test:math:
+      mandatory: 2
+      advisory: 1
+      payload: 2 bytes
   part header size: 16
   part type: "test:ping"
   part id: "4"
   part parameters: 0
   payload chunk size: 0
-  part header size: 0
-  end of bundle2 stream
-  parts count:   5
-    :test:empty:
-      mandatory: 0
-      advisory: 0
-      payload: 0 bytes
-    :test:empty:
-      mandatory: 0
-      advisory: 0
-      payload: 0 bytes
-    :test:song:
-      mandatory: 0
-      advisory: 0
-      payload: 178 bytes
-    :test:math:
-      mandatory: 2
-      advisory: 1
-      payload: 2 bytes
     :test:ping:
       mandatory: 0
       advisory: 0
       payload: 0 bytes
+  part header size: 0
+  end of bundle2 stream
+  parts count:   5
 
 Test actual unbundling of test part
 =======================================
@@ -509,11 +510,11 b' The reply is valid'
 
   $ hg statbundle2 < ../reply.hg2
   options count: 0
-  parts count:   1
     :test:pong:
       mandatory: 1
       advisory: 0
       payload: 0 bytes
+  parts count:   1
 
 Support for changegroup
 ===================================