relnotes: mention the merging of index and nodemap

relnotes/next

@@ -1,23 +1,28 b''
 == New Features ==
 
 
 == New Experimental Features ==
 
 
 == Bug Fixes  ==
 
  * The `indent()` template function was documented to not indent empty lines,
    but it still indented the first line even if it was empty. It no longer does
    that.
 
 == Backwards Compatibility Changes ==
 
 
 == Internal API Changes ==
 
  * Matcher instances no longer have a `explicitdir` property. Consider
    rewriting your code to use `repo.wvfs.isdir()` and/or
    `ctx.hasdir()` instead. Also, the `traversedir` property is now
    also called when only `explicitdir` used to be called. That may
    mean that you can simply remove the use of `explicitdir` if you
    were already using `traversedir`.
+
+ * The `revlog.nodemap` object have been merged into the `revlog.index` object.
+   * `n in revlog.nodemap` becomes `revlog.index.has_node(n)`,
+   * `revlog.nodemap[n]` becomes `revlog.index.rev(n)`,
+   * `revlog.nodemap.get(n)` becomes `revlog.index.get_rev(n)`.