commit: drop duplicated parents

The scripted version of git-commit internally used git-commit-tree which
omitted duplicated parents given from the command line.  This prevented a
nonsensical octopus merge from getting created even when you said "git
merge A B" while you are already on branch A.

However, when git-commit was rewritten in C, this sanity check was lost.
This resurrects it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7502-commit.sh

@@ -226,4 +226,18 @@ test_expect_success 'a SIGTERM should break locks' '
 	! test -f .git/index.lock
 '
 
+rm -f .git/MERGE_MSG .git/COMMIT_EDITMSG
+git reset -q --hard
+
+test_expect_success 'Hand committing of a redundant merge removes dups' '
+
+	git rev-parse second master >expect &&
+	test_must_fail git merge second master &&
+	git checkout master g &&
+	EDITOR=: git commit -a &&
+	git cat-file commit HEAD | sed -n -e "s/^parent //p" -e "/^$/q" >actual &&
+	test_cmp expect actual
+
+'
+
 test_done