bundle2: add an 'idx' argument to the 'getbundle2partsgenerator'

This argument let extensions control in what order bundle2 part are generated
server side during a pull. This is useful to ensure the transaction is in a
proper state before some actions or hooks happens.

mercurial/exchange.py

@@ -1135,7 +1135,7 b' getbundle2partsorder = []'
 # This exists to help extensions wrap steps if necessary
 getbundle2partsmapping = {}
 
-def getbundle2partsgenerator(stepname):
+def getbundle2partsgenerator(stepname, idx=None):
     """decorator for function generating bundle2 part for getbundle
 
     The function is added to the step -> function mapping and appended to the
@@ -1147,7 +1147,10 b' def getbundle2partsgenerator(stepname):'
     def dec(func):
         assert stepname not in getbundle2partsmapping
         getbundle2partsmapping[stepname] = func
-        getbundle2partsorder.append(stepname)
+        if idx is None:
+            getbundle2partsorder.append(stepname)
+        else:
+            getbundle2partsorder.insert(idx, stepname)
         return func
     return dec