revlogv2: simplify and clarify the processing of each entry

As we add more entries and some of them has non trivial processing it seems
useful to make the processing leaner and clearly separated to simplify futures
patches.

Differential Revision: https://phab.mercurial-scm.org/D10773

mercurial/revlogutils/docket.py

@@ -238,18 +238,29 b' def parse_docket(revlog, data, use_pendi'
     """given some docket data return a docket object for the given revlog"""
     header = S_HEADER.unpack(data[: S_HEADER.size])
     offset = S_HEADER.size
-    version_header = header[0]
-    index_uuid_size = header[1]
+
+    iheader = iter(header)
+
+    version_header = next(iheader)
+
+    index_uuid_size = next(iheader)
     index_uuid = data[offset : offset + index_uuid_size]
     offset += index_uuid_size
-    data_uuid_size = header[2]
+
+    data_uuid_size = next(iheader)
     data_uuid = data[offset : offset + data_uuid_size]
     offset += data_uuid_size
-    index_size = header[3]
-    pending_index_size = header[4]
-    data_size = header[5]
-    pending_data_size = header[6]
-    default_compression_header = header[7]
+
+    index_size = next(iheader)
+
+    pending_index_size = next(iheader)
+
+    data_size = next(iheader)
+
+    pending_data_size = next(iheader)
+
+    default_compression_header = next(iheader)
+
     docket = RevlogDocket(
         revlog,
         use_pending=use_pending,