bundle2: move handler validation out of processpart

As part of refactoring bundle part processing let's move handler validation to
its own function.

Differential Revision: https://phab.mercurial-scm.org/D707

mercurial/bundle2.py

@@ -460,14 +460,9 b' def _processchangegroup(op, cg, tr, sour'
     })
     return ret
 
-def _processpart(op, part):
-    """process a single part from a bundle
-
-    The part is guaranteed to have been fully consumed when the function exits
-    (even if an exception is raised)."""
+def _gethandler(op, part):
     status = 'unknown' # used by debug output
     try:
-        try:
             handler = parthandlermapping.get(part.type)
             if handler is None:
                 status = 'unsupported-type'
@@ -503,6 +498,18 b' def _processpart(op, part):'
                 msg.append(' %s\n' % status)
                 op.ui.debug(''.join(msg))
 
+    return handler
+
+def _processpart(op, part):
+    """process a single part from a bundle
+
+    The part is guaranteed to have been fully consumed when the function exits
+    (even if an exception is raised)."""
+    try:
+        handler = _gethandler(op, part)
+        if handler is None:
+            return
+
         # handler is called outside the above try block so that we don't
         # risk catching KeyErrors from anything other than the
         # parthandlermapping lookup (any KeyError raised by handler()