bundle2: capture transaction rollback message output (issue4614)

The output from the transaction rollback was not included into the reply bundle.
It was eventually caught by the usual 'unbundle' output capture and sent to the
client but the result was out of order on the client side. We now capture the
output for the transaction release and transmit it the same way as all other
output.

We should probably rethink the whole output capture things but this would not be
appropriate for stable.

The is still multiple cases were output failed to be properly capture, they will
be fixed in later changesets.

mercurial/exchange.py

@@ -1284,6 +1284,7 def unbundle(repo, cg, heads, source, ur
     r = 0
     # need a transaction when processing a bundle2 stream
     wlock = lock = tr = None
+    recordout = None
     try:
         check_heads(repo, heads, 'uploading changes')
         # push can proceed
@@ -1301,11 +1302,18 def unbundle(repo, cg, heads, source, ur
             except Exception, exc:
                 exc.duringunbundle2 = True
                 if r is not None:
-                    exc._bundle2salvagedoutput = r.salvageoutput()
+                    parts = exc._bundle2salvagedoutput = r.salvageoutput()
+                    repo.ui.pushbuffer(error=True)
+                    def recordout(output):
+                        part = bundle2.bundlepart('output', data=output,
+                                                  mandatory=False)
+                        parts.append(part)
                 raise
         else:
             lock = repo.lock()
             r = changegroup.addchangegroup(repo, cg, source, url)
     finally:
         lockmod.release(tr, lock, wlock)
+        if recordout is not None:
+            recordout(repo.ui.popbuffer())
     return r

tests/test-bundle2-exchange.t

@@ -559,12 +559,12 Doing the actual push: hook abort
   pushing to other
   searching for changes
   pre-close-tip:e7ec4e813ba6 draft 
-  transaction abort!
-  rollback completed
   remote: adding changesets
   remote: adding manifests
   remote: adding file changes
   remote: added 1 changesets with 1 changes to 1 files
+  remote: transaction abort!
+  remote: rollback completed
   abort: pretxnclose.failpush hook exited with status 1
   [255]
 
@@ -575,10 +575,10 Doing the actual push: hook abort
   remote: adding manifests
   remote: adding file changes
   remote: added 1 changesets with 1 changes to 1 files
+  remote: transaction abort!
+  remote: rollback completed
   abort: pretxnclose.failpush hook exited with status 1
   remote: pre-close-tip:e7ec4e813ba6 draft 
-  remote: transaction abort!
-  remote: rollback completed
   [255]
 
   $ hg -R main push http://localhost:$HGPORT2/ -r e7ec4e813ba6
@@ -588,6 +588,8 Doing the actual push: hook abort
   remote: adding manifests
   remote: adding file changes
   remote: added 1 changesets with 1 changes to 1 files
+  remote: transaction abort!
+  remote: rollback completed
   abort: pretxnclose.failpush hook exited with status 1
   [255]