revlog: generate trivial deltas against null revision

To avoid extra memory usage and performance issues with large files,
generate a trivial delta header for deltas against the null revision
rather than calling the usual delta generator.

We append the delta header to meta rather than prepending it to data
to avoid a large allocate and copy.

mercurial/mdiff.py

@@ -245,6 +245,9 b' def patch(a, bin):'
 def get_matching_blocks(a, b):
     return [(d[0], d[2], d[1] - d[0]) for d in bdiff.blocks(a, b)]
 
+def trivialdiffheader(length):
+    return struct.pack(">lll", 0, 0, length)
+
 patches = mpatch.patches
 patchedsize = mpatch.patchedsize
 textdiff = bdiff.bdiff

mercurial/revlog.py

@@ -1087,9 +1087,13 b' class revlog(object):'
             if infocollect is not None:
                 infocollect(nb)
 
-            d = self.revdiff(a, b)
             p = self.parents(nb)
             meta = nb + p[0] + p[1] + lookup(nb)
+            if a == -1:
+                d = self.revision(nb)
+                meta += mdiff.trivialdiffheader(len(d))
+            else:
+                d = self.revdiff(a, b)
             yield changegroup.genchunk("%s%s" % (meta, d))
 
         yield changegroup.closechunk()