completion: complete '--dd'

'--dd' only makes sense for 'git log' and 'git show', so add it to
__git_log_show_options which is referenced in the completion for these
two commands.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -2053,7 +2053,7 @@ __git_log_shortlog_options="
 "
 # Options accepted by log and show
 __git_log_show_options="
-	--diff-merges --diff-merges= --no-diff-merges --remerge-diff
+	--diff-merges --diff-merges= --no-diff-merges --dd --remerge-diff
 "
 
 __git_diff_merges_opts="off none on first-parent 1 separate m combined c dense-combined cc remerge r"