lib-log-graph: move check_graph function

lib-log-graph: move check_graph function

check_graph is a function shared in the test files t4215 and t6016 used
to format the output graph, but instead of being in a file called by
both test, the function code is repeated in each file.

Move check_graph to lib-log-graph.sh file which both tests already
import graph functions from, renaming it to lib_test_check_graph.

This function is needed for the following commit which includes graph
tests in a new file and requires check_graph.

Mentored-by: Karthik Nayak <karthik.188@gmail.com>
Mentored-by: Chandra Pratap <chandrapratap3519@gmail.com>
Signed-off-by: Pablo Sabater <pabloosabaterr@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>