Documentation/gitattributes: Add subsection header for each attribute

This makes attributes easier to find; before this patch some
attributes had individual subsections, and some didn't.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jnareb

Documentation/gitattributes.txt

@@ -213,6 +213,9 @@ with `crlf`, and then `ident` and fed to `filter`.
 Generating diff text
 ~~~~~~~~~~~~~~~~~~~~
 
+`diff`
+^^^^^^
+
 The attribute `diff` affects if 'git-diff' generates textual
 patch for the path or just says `Binary files differ`.  It also
 can affect what line is shown on the hunk header `@@ -k,l +n,m @@`
@@ -323,6 +326,9 @@ patterns are available:
 Performing a three-way merge
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
+`merge`
+^^^^^^^
+
 The attribute `merge` affects how three versions of a file is
 merged when a file-level merge is necessary during `git merge`,
 and other programs such as `git revert` and `git cherry-pick`.