revlog: give EXTSTORED flag value to narrowhg

Narrowhg has been using "1 << 14" as its revlog flag value for a long
time. We (Google) have many repos with that value in production
already. When the same value was reserved for EXTSTORED, it made those
repos invalid. Upgrading them will be a little painful. We should
clearly have reserved the value for narrowhg a long time ago. Since
the EXTSTORED flag is not yet in any release and Facebook also says
they have not started using it in production, so it should be okay to
change it. This patch gives the current value (1 << 14) back to
narrowhg and gives a new value (1 << 13) to EXTSTORED.

mercurial/help/internals/revlogs.txt

@@ -94,7 +94,10 b' 6-7 (2 bytes)'
 
    0: REVIDX_ISCENSORED revision has censor metadata, must be verified.
 
-   1: REVIDX_EXTSTORED revision data is stored externally.
+   1: REVIDX_ELLIPSIS revision hash does not match its data. Used by
+   narrowhg
+
+   2: REVIDX_EXTSTORED revision data is stored externally.
 
 8-11 (4 bytes)
    Compressed length of revision data / chunk as stored in revlog.

mercurial/revlog.py

@@ -54,11 +54,13 b' REVLOGNG_FLAGS = REVLOGNGINLINEDATA | RE'
 
 # revlog index flags
 REVIDX_ISCENSORED = (1 << 15) # revision has censor metadata, must be verified
-REVIDX_EXTSTORED = (1 << 14) # revision data is stored externally
+REVIDX_ELLIPSIS = (1 << 14) # revision hash does not match data (narrowhg)
+REVIDX_EXTSTORED = (1 << 13) # revision data is stored externally
 REVIDX_DEFAULT_FLAGS = 0
 # stable order in which flags need to be processed and their processors applied
 REVIDX_FLAGS_ORDER = [
     REVIDX_ISCENSORED,
+    REVIDX_ELLIPSIS,
     REVIDX_EXTSTORED,
 ]
 REVIDX_KNOWN_FLAGS = util.bitsfrom(REVIDX_FLAGS_ORDER)