bundle2: also capture hook output during processing

External hook used to directly write on stdout and stderr. As a result their
output was not captured by the bundle2 processing. This resulted in confusing
out of order output on the client side. We are now capturing hooks output in
this context.

mercurial/bundle2.py

@@ -354,7 +354,7 b' def _processpart(op, part):'
         # itself represents a defect of a different variety).
         output = None
         if op.reply is not None:
-            op.ui.pushbuffer(error=True)
+            op.ui.pushbuffer(error=True, subproc=True)
             output = ''
         try:
             handler(op, part)

mercurial/exchange.py

@@ -1303,7 +1303,7 b' def unbundle(repo, cg, heads, source, ur'
                 exc.duringunbundle2 = True
                 if r is not None:
                     parts = exc._bundle2salvagedoutput = r.salvageoutput()
-                    repo.ui.pushbuffer(error=True)
+                    repo.ui.pushbuffer(error=True, subproc=True)
                     def recordout(output):
                         part = bundle2.bundlepart('output', data=output,
                                                   mandatory=False)

tests/test-bundle2-exchange.t

@@ -549,6 +549,7 b' Doing the actual push: hook abort'
   > reason =
   > [hooks]
   > pretxnclose.failpush = false
+  > txnabort.failpush = echo 'Cleaning up the mess...'
   > EOF
 
   $ "$TESTDIR/killdaemons.py" $DAEMON_PIDS
@@ -564,6 +565,7 b' Doing the actual push: hook abort'
   remote: adding file changes
   remote: added 1 changesets with 1 changes to 1 files
   remote: transaction abort!
+  remote: Cleaning up the mess...
   remote: rollback completed
   abort: pretxnclose.failpush hook exited with status 1
   [255]
@@ -576,6 +578,7 b' Doing the actual push: hook abort'
   remote: adding file changes
   remote: added 1 changesets with 1 changes to 1 files
   remote: transaction abort!
+  remote: Cleaning up the mess...
   remote: rollback completed
   abort: pretxnclose.failpush hook exited with status 1
   remote: pre-close-tip:e7ec4e813ba6 draft 
@@ -589,6 +592,7 b' Doing the actual push: hook abort'
   remote: adding file changes
   remote: added 1 changesets with 1 changes to 1 files
   remote: transaction abort!
+  remote: Cleaning up the mess...
   remote: rollback completed
   abort: pretxnclose.failpush hook exited with status 1
   [255]