completion: complete --line-prefix

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1757,7 +1757,7 @@ __git_diff_common_options="--stat --numstat --shortstat --summary
 			--textconv --no-textconv --break-rewrites
 			--patch --no-patch --cc --combined-all-paths
 			--anchored= --compact-summary --ignore-matching-lines=
-			--irreversible-delete
+			--irreversible-delete --line-prefix
 "
 
 # Options for diff/difftool