ensure the no-tab rule

* HACKING: Mention the no-tab rule.
* tests/sanity/style.test: Only test for it at the beginning of line.

HACKING

@@ -278,6 +278,9 @@ Encoding
 
   * Use UTF-8 for non-ASCII characters.
 
+  * Do not use tabs for indentation in C++ files.  Use only space to
+    prevent issues with people assuming different tab widths.
+
   * If you edit files encoded in Latin-1 (the original default
     encoding for the project), feel free to convert them to UTF-8.
     In emacs the simplest way to convert the file is to add a comment

tests/sanity/style.test

@@ -289,8 +289,8 @@ for dir in "$TOP/spot" "$TOP/bin" "$TOP/tests"; do
 	    $GREP '#.*include.*<iostream>' $tmp &&
 	      diag 'Avoid <iostream> in headers, better use <iosfwd>.'
 	  fi
-    e$GREP '	' $tmp &&
-	diag 'Use spaces instead of tabular.'
+          $GREP '^[ ]*	' $tmp &&
+              diag 'Use spaces instead of tabs.'
 	  # Headers from spot/priv/ are not installed, so may only be
 	  # included from *.cc files or from other spot/priv/ headers
 	  # (in the latter case they do not have to specify the priv/