sidedata: add a function for _processflagsraw usage

Entry with side data cannot directly validate their rawtext

Differential Revision: https://phab.mercurial-scm.org/D6892

mercurial/revlogutils/sidedata.py

@@ -73,3 +73,7 b' def sidedatareadprocessor(rl, text):'
         dataoffset = nextdataoffset
     text = text[dataoffset:]
     return text, True, sidedata
+
+def sidedatarawprocessor(rl, text):
+    # side data modifies rawtext and prevent rawtext hash validation
+    return False