xz-utils/README.Debian: flesh out "differences from upstream"

In addition to updating the list of patches applied, also list patches
not applied to make the relationship to the upstream release
5.1.2alpha crystal clear.
diff --git a/debian/xz-utils.README.Debian b/debian/xz-utils.README.Debian
index 180be4c..7c0916c 100644
--- a/debian/xz-utils.README.Debian
+++ b/debian/xz-utils.README.Debian
@@ -48,6 +48,15 @@
   Check that the first byte of range encoded data is zero to catch
   broken files sooner.
 
+Changes in 5.1.2alpha not applied:
+
+  Docs: Language fix to 01_compress_easy.c
+  xz: Add incomplete support for --block-list
+  INSTALL: Document --enable-symbol-versions
+  configure: Add a comment about *-linux tuples for clarity
+  TODO: Warn that threads and fork() do not mix well
+  Bump the version number and update NEWS for 5.1.2alpha
+
 LZMA Utils compatibility
 ------------------------