revlog: support importing censored file revision tombstones

This change allows a revision log to not fail integrity checks when applying a
changegroup delta (eg from a bundle) results in a censored file tombstone. The
tombstone is inserted as-is, so future integrity verification will observe the
tombstone. Deltas based on the tombstone will also remain correct.

The new code path is encountered for *exactly* the cases where _addrevision is
importing a tombstone from a changegroup. When committing a file containing
the "magic" tombstone text, the "text" parameter will be non-empty and the
checkhash call is not executed (and when committing, the node will be computed
to match the "magic" tombstone text).

mercurial/revlog.py

@@ -42,6 +42,7 b' REVIDX_KNOWN_FLAGS = 0'
 
 RevlogError = error.RevlogError
 LookupError = error.LookupError
+CensoredNodeError = error.CensoredNodeError
 
 def getoffset(q):
     return int(q >> 16)
@@ -1176,7 +1177,10 b' class revlog(object):'
             ifh.flush()
             basetext = self.revision(self.node(cachedelta[0]))
             btext[0] = mdiff.patch(basetext, cachedelta[1])
+            try:
             self.checkhash(btext[0], p1, p2, node)
+            except CensoredNodeError:
+                pass # always import a censor tombstone.
             return btext[0]
 
         def builddelta(rev):