use strchrnul() in place of strchr() and strlen()

use strchrnul() in place of strchr() and strlen()

Avoid scanning strings twice, once with strchr() and then with
strlen(), by using strchrnul().

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Rohit Mani <rohit.mani@outlook.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>