completion: complete diff --word-diff

Signed-off-by: NRamkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1246,7 +1246,7 @@ __git_diff_common_options="--stat --numstat --shortstat --summary
 			--no-prefix --src-prefix= --dst-prefix=
 			--inter-hunk-context=
 			--patience --histogram --minimal
-			--raw
+			--raw --word-diff
 			--dirstat --dirstat= --dirstat-by-file
 			--dirstat-by-file= --cumulative
 			--diff-algorithm=