Enable the markdown hunk header for diff context.

Telling Git that .md files are markdown makes the @@ context lines in
git-diff show the section heading that contains the change, rather than
just showing one additional line of the context before the change.

For more information, see the section "Defining a custom hunk-header" in
the gitattributes(5) man page.

Author: amartini51

.gitattributes

@@ -1,2 +1,3 @@
 *.md linguist-detectable=true
 *.md linguist-documentation=false
+*.md diff=markdown