diff: --ignore-blank-lines was too enthusiastic

It was ignoring changes from:

ab

to:

a
b

mercurial/mdiff.py

@@ -72,7 +72,7 b' def wsclean(opts, text, blank=True):'
         text = re.sub('[ \t\r]+', ' ', text)
         text = text.replace(' \n', '\n')
     if blank and opts.ignoreblanklines:
-        text = re.sub('\n+', '', text)
+        text = re.sub('\n+', '\n', text).strip('\n')
     return text
 
 def diffline(revs, a, b, opts):

tests/test-diff-ignore-whitespace.t

@@ -442,3 +442,16 b' Only new line noticed:'
 New line not noticed when space change ignored:
 
   $ hg ndiff --ignore-blank-lines --ignore-all-space
+
+Do not ignore all newlines, only blank lines
+
+  $ printf 'hello \nworld\ngoodbye world\n' > foo
+  $ hg ndiff --ignore-blank-lines
+  diff -r 540c40a65b78 foo
+  --- a/foo
+  +++ b/foo
+  @@ -1,2 +1,3 @@
+  -hello world
+  +hello 
+  +world
+   goodbye world