check-commit: check for double-addition of blank lines

Previously, we were only checking for a blank line being added next to
an existing one. Now we also check for two being added at the same time.

contrib/check-commit

@@ -34,7 +34,7 errors = [
      "summary keyword should be most user-relevant one-word command or topic"),
     (afterheader + r".*\.\s*\n", "don't add trailing period on summary line"),
     (afterheader + r".{79,}", "summary line too long (limit is 78)"),
-    (r"\n\+\n \n", "adds double empty line"),
+    (r"\n\+\n( |\+)\n", "adds double empty line"),
     (r"\n \n\+\n", "adds double empty line"),
     (r"\n\+[ \t]+def [a-z]+_[a-z]", "adds a function with foo_bar naming"),
 ]

tests/test-contrib-check-commit.t

@@ -87,6 +87,10 A patch with other errors:
   > @@ -599,7 +599,7 @@
   >          if opts.get('all'):
   >  
+  > 
+  > +
+  > + some = otherjunk
+  > +
   > +
   > + def blah_blah(x):
   > +     pass
@@ -102,10 +106,10 A patch with other errors:
    This has no topic and ends with a period.
   7: don't add trailing period on summary line
    This has no topic and ends with a period.
-  15: adds double empty line
-   +
-  16: adds a function with foo_bar naming
-   + def blah_blah(x):
   19: adds double empty line
    +
+  20: adds a function with foo_bar naming
+   + def blah_blah(x):
+  23: adds double empty line
+   +
   [1]