unionrevlog: extract 'baserevision' and 'baserevdiff' methods

This makes possible to use unionrevlog class with subclasses of revlog that
override revlog's 'revision' and 'revdiff' methods. In particular this change
is necessary to implement manifest compression, as it allows extension to
replace manifest class and override 'revision' amd 'revdiff' methods there.

mercurial/unionrepo.py

@@ -70,7 +70,7 b' class unionrevlog(revlog.revlog):'
                 self.revlog2.rev(self.node(rev1)),
                 self.revlog2.rev(self.node(rev2)))
         elif rev1 <= self.repotiprev and rev2 <= self.repotiprev:
-            return revlog.revlog.revdiff(self, rev1, rev2)
+            return self.baserevdiff(rev1, rev2)
 
         return mdiff.textdiff(self.revision(self.node(rev1)),
                               self.revision(self.node(rev2)))
@@ -93,10 +93,20 b' class unionrevlog(revlog.revlog):'
             text = self.revlog2.revision(node)
             self._cache = (node, rev, text)
         else:
-            text = revlog.revlog.revision(self, rev)
+            text = self.baserevision(rev)
             # already cached
         return text
 
+    def baserevision(self, nodeorrev):
+        # Revlog subclasses may override 'revision' method to modify format of
+        # content retrieved from revlog. To use unionrevlog with such class one
+        # needs to override 'baserevision' and make more specific call here.
+        return revlog.revlog.revision(self, nodeorrev)
+
+    def baserevdiff(self, rev1, rev2):
+        # Exists for the same purpose as baserevision.
+        return revlog.revlog.revdiff(self, rev1, rev2)
+
     def addrevision(self, text, transaction, link, p1=None, p2=None, d=None):
         raise NotImplementedError
     def addgroup(self, revs, linkmapper, transaction):
@@ -114,6 +124,15 b' class unionchangelog(unionrevlog, change'
         unionrevlog.__init__(self, opener, self.indexfile, changelog2,
                              linkmapper)
 
+    def baserevision(self, nodeorrev):
+        # Although changelog doesn't override 'revision' method, some extensions
+        # may replace this class with another that does. Same story with
+        # manifest and filelog classes.
+        return changelog.changelog.revision(self, nodeorrev)
+
+    def baserevdiff(self, rev1, rev2):
+        return changelog.changelog.revdiff(self, rev1, rev2)
+
 class unionmanifest(unionrevlog, manifest.manifest):
     def __init__(self, opener, opener2, linkmapper):
         manifest.manifest.__init__(self, opener)
@@ -121,6 +140,12 b' class unionmanifest(unionrevlog, manifes'
         unionrevlog.__init__(self, opener, self.indexfile, manifest2,
                              linkmapper)
 
+    def baserevision(self, nodeorrev):
+        return manifest.manifest.revision(self, nodeorrev)
+
+    def baserevdiff(self, rev1, rev2):
+        return manifest.manifest.revdiff(self, rev1, rev2)
+
 class unionfilelog(unionrevlog, filelog.filelog):
     def __init__(self, opener, path, opener2, linkmapper, repo):
         filelog.filelog.__init__(self, opener, path)
@@ -129,6 +154,12 b' class unionfilelog(unionrevlog, filelog.'
                              linkmapper)
         self._repo = repo
 
+    def baserevision(self, nodeorrev):
+        return filelog.filelog.revision(self, nodeorrev)
+
+    def baserevdiff(self, rev1, rev2):
+        return filelog.filelog.revdiff(self, rev1, rev2)
+
     def _file(self, f):
         self._repo.file(f)