t1300: extract and use test_cmp_config()

t1300: extract and use test_cmp_config()

In many config-related tests it's common to check if a config variable
has expected value and we want to print the differences when the test
fails. Doing it the normal way is three lines of shell code. Let's add
a function do to all this (and a little more).

This function has uses outside t1300 as well but I'm not going to
convert them all. And it will be used in the next commit where
per-worktree config feature is introduced.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>