revlog: rename `versionflags` to header

The variable is named as such because it contains "version" information and
"flags" information. However you mostly needs to know the code to understand the
name which is not great. The fact that this is the very first four bytes in all
revlog seems more relevant, so we rename the variable "header".

Differential Revision: https://phab.mercurial-scm.org/D10596

mercurial/revlog.py

@@ -485,13 +485,13 b' class revlog(object):'
         self._initempty = True
         indexdata = self._get_data(self._indexfile, mmapindexthreshold)
         if len(indexdata) > 0:
-            versionflags = INDEX_HEADER.unpack(indexdata[:4])[0]
+            header = INDEX_HEADER.unpack(indexdata[:4])[0]
             self._initempty = False
         else:
-            versionflags = newversionflags
-
-        flags = self._format_flags = versionflags & ~0xFFFF
-        fmt = self._format_version = versionflags & 0xFFFF
+            header = newversionflags
+
+        flags = self._format_flags = header & ~0xFFFF
+        fmt = self._format_version = header & 0xFFFF
 
         if fmt == REVLOGV0:
             if flags:
@@ -510,8 +510,8 b' class revlog(object):'
                     % (flags >> 16, fmt, self.display_id)
                 )
 
-            self._inline = versionflags & FLAG_INLINE_DATA
-            self._generaldelta = versionflags & FLAG_GENERALDELTA
+            self._inline = header & FLAG_INLINE_DATA
+            self._generaldelta = header & FLAG_GENERALDELTA
 
         elif fmt == REVLOGV2:
             if flags & ~REVLOGV2_FLAGS: