revlog: support writing generaldelta revlogs

With generaldelta switched on, deltas are always computed against the first
parent when adding revisions. This is done regardless of what revision the
incoming bundle, if any, is deltaed against.

The exact delta building strategy is subject to change, but this will not
affect compatibility.

Generaldelta is switched off by default.

mercurial/localrepo.py

@@ -21,7 +21,7 b' propertycache = util.propertycache'
 class localrepository(repo.repository):
     capabilities = set(('lookup', 'changegroupsubset', 'branchmap', 'pushkey',
                         'known', 'getbundle'))
-    supportedformats = set(('revlogv1',))
+    supportedformats = set(('revlogv1', 'generaldelta'))
     supported = supportedformats | set(('store', 'fncache', 'shared',
                                         'dotencode'))
 
@@ -61,6 +61,8 b' class localrepository(repo.repository):'
                         '\0\0\0\2' # represents revlogv2
                         ' dummy changelog to prevent using the old repo layout'
                     )
+                if self.ui.configbool('format', 'generaldelta', False):
+                    requirements.append("generaldelta")
             else:
                 raise error.RepoError(_("repository %s not found") % path)
         elif create:
@@ -115,6 +117,8 b' class localrepository(repo.repository):'
     def _applyrequirements(self, requirements):
         self.requirements = requirements
         self.sopener.options = {}
+        if 'generaldelta' in requirements:
+            self.sopener.options['generaldelta'] = 1
 
     def _writerequirements(self):
         reqfile = self.opener("requires", "w")

mercurial/revlog.py

@@ -226,10 +226,13 b' class revlog(object):'
         self._nodepos = None
 
         v = REVLOG_DEFAULT_VERSION
-        if hasattr(opener, 'options') and 'defversion' in opener.options:
+        if hasattr(opener, 'options'):
+            if 'defversion' in opener.options:
             v = opener.options['defversion']
             if v & REVLOGNG:
                 v |= REVLOGNGINLINEDATA
+            if v & REVLOGNG and 'generaldelta' in opener.options:
+                v |= REVLOGGENERALDELTA
 
         i = ''
         try:
@@ -1003,10 +1006,14 b' class revlog(object):'
             l = len(data[1]) + len(data[0])
             basecache = self._basecache
             if basecache and basecache[0] == rev:
-                base = basecache[1]
+                chainbase = basecache[1]
             else:
-                base = self.chainbase(rev)
-            dist = l + offset - self.start(base)
+                chainbase = self.chainbase(rev)
+            dist = l + offset - self.start(chainbase)
+            if self._generaldelta:
+                base = rev
+            else:
+                base = chainbase
             return dist, l, data, base
 
         curr = len(self)
@@ -1019,6 +1026,9 b' class revlog(object):'
 
         # should we try to build a delta?
         if prev != nullrev:
+            if self._generaldelta:
+                d = builddelta(p1r)
+            else:
             d = builddelta(prev)
             dist, l, data, base = d