revlog: reorder index accessors to match data structure order

Index entries are ordered tuples. We have accessors in the revlog
class to map tuple offsets to names. To help reinforce the order,
reorder the methods so they match the order of elements in the
tuple. While I'm here, also sneak in some minimal documentation.

mercurial/revlog.py

@@ -380,22 +380,29 class revlog(object):
                     return r
             raise LookupError(node, self.indexfile, _('no node'))
 
-    def node(self, rev):
-        return self.index[rev][7]
-    def linkrev(self, rev):
-        return self.index[rev][4]
-    def parents(self, node):
-        i = self.index
-        d = i[self.rev(node)]
-        return i[d[5]][7], i[d[6]][7] # map revisions to nodes inline
-    def parentrevs(self, rev):
-        return self.index[rev][5:7]
+    # Accessors for index entries.
+
+    # First tuple entry is 8 bytes. First 6 bytes are offset. Last 2 bytes
+    # are flags.
     def start(self, rev):
         return int(self.index[rev][0] >> 16)
-    def end(self, rev):
-        return self.start(rev) + self.length(rev)
+
+    def flags(self, rev):
+        return self.index[rev][0] & 0xFFFF
+
     def length(self, rev):
         return self.index[rev][1]
+
+    def rawsize(self, rev):
+        """return the length of the uncompressed text for a given revision"""
+        l = self.index[rev][2]
+        if l >= 0:
+            return l
+
+        t = self.revision(self.node(rev))
+        return len(t)
+    size = rawsize
+
     def chainbase(self, rev):
         base = self._chainbasecache.get(rev)
         if base is not None:
@@ -409,6 +416,26 class revlog(object):
 
         self._chainbasecache[rev] = base
         return base
+
+    def linkrev(self, rev):
+        return self.index[rev][4]
+
+    def parentrevs(self, rev):
+        return self.index[rev][5:7]
+
+    def node(self, rev):
+        return self.index[rev][7]
+
+    # Derived from index values.
+
+    def end(self, rev):
+        return self.start(rev) + self.length(rev)
+
+    def parents(self, node):
+        i = self.index
+        d = i[self.rev(node)]
+        return i[d[5]][7], i[d[6]][7] # map revisions to nodes inline
+
     def chainlen(self, rev):
         return self._chaininfo(rev)[0]
 
@@ -478,18 +505,6 class revlog(object):
         chain.reverse()
         return chain, stopped
 
-    def flags(self, rev):
-        return self.index[rev][0] & 0xFFFF
-    def rawsize(self, rev):
-        """return the length of the uncompressed text for a given revision"""
-        l = self.index[rev][2]
-        if l >= 0:
-            return l
-
-        t = self.revision(self.node(rev))
-        return len(t)
-    size = rawsize
-
     def ancestors(self, revs, stoprev=0, inclusive=False):
         """Generate the ancestors of 'revs' in reverse topological order.
         Does not generate revs lower than stoprev.