revlog: REVIDX_EXTSTORED flag

This flag will be used by the lfs extension to mark the revision data as stored
externally.

mercurial/help/internals/revlogs.txt

@@ -90,6 +90,7 b' 0-5 (6 bytes)'
 6-7 (2 bytes)
    Bit flags impacting revision behavior. The following bit offsets define:
    0: REVIDX_ISCENSORED revision has censor metadata, must be verified.
+   1: REVIDX_EXTSTORED revision data is stored externally.
 8-11 (4 bytes)
    Compressed length of revision data / chunk as stored in revlog.
 12-15 (4 bytes)

mercurial/revlog.py

@@ -54,10 +54,12 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_DEFAULT_FLAGS = 0
 # stable order in which flags need to be processed and their processors applied
 REVIDX_FLAGS_ORDER = [
     REVIDX_ISCENSORED,
+    REVIDX_EXTSTORED,
 ]
 REVIDX_KNOWN_FLAGS = util.bitsfrom(REVIDX_FLAGS_ORDER)