bundle2.unbundlepart: decouple mandatory from parttype

Encoding whether or not a part is mandatory in the capitalization of the
parttype is unintuitive and error-prone.  This sequence of patches separates
these concerns in the API to reduce programmer error and pave the way for
a potential change in how this information is transmitted over the wire.

This patch separates the two pieces of information when reading the part header
so that it's unnecessary to know how they were combined during transmission.

mercurial/bundle2.py

@@ -334,7 +334,7 b' def _processpart(op, part):'
                 raise error.UnsupportedPartError(parttype=key,
                                                params=unknownparams)
         except error.UnsupportedPartError, exc:
-            if key != parttype: # mandatory parts
+            if part.mandatory: # mandatory parts
                 raise
             op.ui.debug('ignoring unsupported advisory part %s\n' % exc)
             return # skip to part processing
@@ -786,6 +786,7 b' class unbundlepart(unpackermixin):'
         self.mandatorykeys = ()
         self._payloadstream = None
         self._readheader()
+        self._mandatory = None
 
     def _fromheader(self, size):
         """return the next <size> byte from the header"""
@@ -818,6 +819,9 b' class unbundlepart(unpackermixin):'
         self.ui.debug('part type: "%s"\n' % self.type)
         self.id = self._unpackheader(_fpartid)[0]
         self.ui.debug('part id: "%s"\n' % self.id)
+        # extract mandatory bit from type
+        self.mandatory = (self.type != self.type.lower())
+        self.type = self.type.lower()
         ## reading parameters
         # param count
         mancount, advcount = self._unpackheader(_fpartparamcount)