completion: add --anchored to diff's options

This flag was introduced in 2477ab2e (diff: support anchoring line(s),
2017-11-27) but back then, the bash completion script did not learn
about the new flag. Add it.

Signed-off-by: Thomas Braun <thomas.braun@virtuell-zuhause.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1729,6 +1729,7 @@ __git_diff_common_options="--stat --numstat --shortstat --summary
 			--indent-heuristic --no-indent-heuristic
 			--textconv --no-textconv
 			--patch --no-patch
+			--anchored=
 "
 
 __git_diff_difftool_options="--cached --staged --pickaxe-all --pickaxe-regex