test-revlog: adds a new root revision with a delta against nullrev

We add a revision with null parent but that is not the first revision of the
revlog. It make it a different a case that is worthy of testing.

tests/bundles/test-revlog-diff-relative-to-nullrev.sh

@@ -29,6 +29,9 b' echo hi > a'
 ../../../hg commit -Am root-B
 echo ho > a
 ../../../hg commit -Am child-A
+hg up null
+echo ha > a
+../../../hg commit -Am root-A
 ../../../hg debugdeltachain a
 rm -rf .hg/cache/ .hg/wcache/
 cd ..

tests/test-revlog.t

@@ -79,8 +79,11 b' This is what we did to produce the repo '
       rev      p1      p2  chain# chainlen     prev   delta       size    rawsize  chainsize     ratio   lindist extradist extraratio   readsize largestblk rddensity srchunks
         0      -1      -1       1        2       -1      p1         15          3         15   5.00000        15         0    0.00000         15         15   1.00000        1
         1       0      -1       1        2       -1      p2         15          3         15   5.00000        30        15    1.00000         30         30   0.50000        1
+        2      -1      -1       1        2       -1      p1         15          3         15   5.00000        45        30    2.00000         45         45   0.33333        1
   $ hg cat --config rhg.cat=true -r 0 a
   hi
   $ hg cat --config rhg.cat=true -r 1 a
   ho
+  $ hg cat --config rhg.cat=true -r 2 a
+  ha
   $ cd ..