revlog: support reading generaldelta revlogs

Generaldelta is a new revlog global flag. When it's turned on, the base field
of each revision entry holds the deltaparent instead of the base revision of
the current delta chain.

This allows for great potential flexibility when generating deltas, as any
revision can serve as deltaparent. Previously, the deltaparent for revision r
was hardcoded to be r - 1.

The base revision of the delta chain can still be accessed as before, since it
is now computed in an iterative fashion, following the deltaparents backwards.

mercurial/revlog.py

@@ -27,10 +27,11 b' import struct, zlib, errno'
 REVLOGV0 = 0
 REVLOGNG = 1
 REVLOGNGINLINEDATA = (1 << 16)
+REVLOGGENERALDELTA = (1 << 17)
 REVLOG_DEFAULT_FLAGS = REVLOGNGINLINEDATA
 REVLOG_DEFAULT_FORMAT = REVLOGNG
 REVLOG_DEFAULT_VERSION = REVLOG_DEFAULT_FORMAT | REVLOG_DEFAULT_FLAGS
-REVLOGNG_FLAGS = REVLOGNGINLINEDATA
+REVLOGNG_FLAGS = REVLOGNGINLINEDATA | REVLOGGENERALDELTA
 
 # revlog index flags
 REVIDX_KNOWN_FLAGS = 0
@@ -243,6 +244,7 b' class revlog(object):'
 
         self.version = v
         self._inline = v & REVLOGNGINLINEDATA
+        self._generaldelta = v & REVLOGGENERALDELTA
         flags = v & ~0xFFFF
         fmt = v & 0xFFFF
         if fmt == REVLOGV0 and flags:
@@ -830,8 +832,11 b' class revlog(object):'
 
     def deltaparent(self, rev):
         """return deltaparent of the given revision"""
-        if self.index[rev][3] == rev:
+        base = self.index[rev][3]
+        if base == rev:
             return nullrev
+        elif self._generaldelta:
+            return base
         else:
             return rev - 1
 
@@ -865,11 +870,15 b' class revlog(object):'
         # build delta chain
         chain = []
         index = self.index # for performance
+        generaldelta = self._generaldelta
         iterrev = rev
         e = index[iterrev]
         while iterrev != e[3] and iterrev != cachedrev:
             chain.append(iterrev)
-            iterrev -= 1
+            if generaldelta:
+                iterrev = e[3]
+            else:
+                iterrev -= 1
             e = index[iterrev]
         chain.reverse()
         base = iterrev