t4215: use helper function to check output

t4215: use helper function to check output

When git commands are placed in the upstream of a pipe, their return
codes are lost. In this particular case, it is especially bad since we
are testing the intricacies of `git log --graph` behavior and if we hit
an unexpected failure or segfault, we want to know this.

Extract the common output checking logic into check_graph() where we
redirect the output of git commands upstream of pipe into a file and
have sed read from that file so that git failures are detected.

This patch is best viewed with `--color-moved`.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>