revlog: move the "index header" struct inside revlog.utils.constants The struct was previous called "version", but this is actually "version" + "flags". So header seems like a better name. The move to the `constants` module has the same motivation as the INDEX_ENTRY_V# ones. Differential Revision: https://phab.mercurial-scm.org/D10306

--- a/mercurial/revlogutils/constants.py	Mon Apr 05 12:21:12 2021 +0200
+++ b/mercurial/revlogutils/constants.py	Mon Apr 05 12:21:23 2021 +0200
@@ -15,6 +15,8 @@
 
 ### main revlog header
 
+INDEX_HEADER = struct.Struct(b">I")
+
 ## revlog version
 REVLOGV0 = 0
 REVLOGV1 = 1