index: use an explicit constant for INDEX_HEADER format and use it for docket

This avoid leaking python-3.6 compatibility details too much.

Differential Revision: https://phab.mercurial-scm.org/D10831

mercurial/revlogutils/constants.py

@@ -29,7 +29,9 b' ALL_KINDS = {'
 
 ### main revlog header
 
-INDEX_HEADER = struct.Struct(b">I")
+# We cannot rely on  Struct.format is inconsistent for python <=3.6 versus above
+INDEX_HEADER_FMT = b">I"
+INDEX_HEADER = struct.Struct(INDEX_HEADER_FMT)
 
 ## revlog version
 REVLOGV0 = 0

mercurial/revlogutils/docket.py

@@ -95,9 +95,7 b' if stable_docket_file:'
 # * 8 bytes: size of data
 # * 8 bytes: pending size of data
 # * 1 bytes: default compression header
-S_HEADER = struct.Struct(
-    pycompat.sysstr(constants.INDEX_HEADER.format) + 'BBLLLLc'
-)
+S_HEADER = struct.Struct(constants.INDEX_HEADER_FMT + b'BBLLLLc')
 
 
 class RevlogDocket(object):