sequencer: unset GIT_CHERRY_PICK_HELP for 'exec' commands

Running "git cherry-pick" as an x-command in the rebase plan loses
the original authorship information.

To fix this, unset GIT_CHERRY_PICK_HELP for 'exec' commands.

Helped-by: Phillip Wood <phillip.wood123@gmail.com>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -3647,6 +3647,7 @@ static int do_exec(struct repository *r, const char *command_line)
 	fprintf(stderr, _("Executing: %s\n"), command_line);
 	cmd.use_shell = 1;
 	strvec_push(&cmd.args, command_line);
+	strvec_push(&cmd.env, "GIT_CHERRY_PICK_HELP");
 	status = run_command(&cmd);
 
 	/* force re-reading of the cache */

t/t3404-rebase-interactive.sh

@@ -153,6 +153,18 @@ test_expect_success 'rebase -i with the exec command checks tree cleanness' '
 	git rebase --continue
 '
 
+test_expect_success 'cherry-pick works with rebase --exec' '
+	test_when_finished "git cherry-pick --abort; \
+			    git rebase --abort; \
+			    git checkout primary" &&
+	echo "exec git cherry-pick G" >todo &&
+	(
+		set_replace_editor todo &&
+		test_must_fail git rebase -i D D
+	) &&
+	test_cmp_rev G CHERRY_PICK_HEAD
+'
+
 test_expect_success 'rebase -x with empty command fails' '
 	test_when_finished "git rebase --abort ||:" &&
 	test_must_fail env git rebase -x "" @ 2>actual &&