bundle2: add generic debug output regarding processed bundle

If we are about to hide the detailed debug output, we need some generic debug
message to replace it in a concise way.

mercurial/bundle2.py

@@ -318,6 +318,16 b' def processbundle(repo, unbundler, trans'
     # - replace this is a init function soon.
     # - exception catching
     unbundler.params
+    if repo.ui.debugflag:
+        msg = ['bundle2-input-bundle:']
+        if unbundler.params:
+            msg.append(' %i params')
+        if op.gettransaction is None:
+            msg.append(' no-transaction')
+        else:
+            msg.append(' with-transaction')
+        msg.append('\n')
+        repo.ui.debug(''.join(msg))
     iterparts = unbundler.iterparts()
     part = None
     try:

tests/test-bundle2-format.t

@@ -550,6 +550,7 b' Process the bundle'
   $ hg unbundle2 --debug --config progress.debug=true < ../parts.hg2
   bundle2-input: start processing of HG20 stream
   bundle2-input: reading bundle2 stream parameters
+  bundle2-input-bundle: with-transaction
   bundle2-input: start extraction of bundle2 parts
   bundle2-input: part header size: 17
   bundle2-input: part type: "test:empty"