revlog: define censored flag for revlogng index

This flag bit will be used to cheaply signal censorship presence to upper
layers (exchange, verify). It indicates that censorship metadata is present
but does not attest to the verifiability of that metadata.

For the censorship design, see: http://mercurial.selenic.com/wiki/CensorPlan

mercurial/revlog.py

@@ -34,7 +34,9 b' REVLOG_DEFAULT_VERSION = REVLOG_DEFAULT_'
 REVLOGNG_FLAGS = REVLOGNGINLINEDATA | REVLOGGENERALDELTA
 
 # revlog index flags
-REVIDX_KNOWN_FLAGS = 0
+REVIDX_ISCENSORED = (1 << 15) # revision has censor metadata, must be verified
+REVIDX_DEFAULT_FLAGS = 0
+REVIDX_KNOWN_FLAGS = REVIDX_ISCENSORED
 
 # max size of revlog with inline data
 _maxinline = 131072