t/helper/test-read-graph.c: support commit-graph chains

t/helper/test-read-graph.c: support commit-graph chains

In 61df89c8e5 (commit-graph: don't early exit(1) on e.g. "git status",
2019-03-25), the former 'load_commit_graph_one' was refactored into
'open_commit_graph' and 'load_commit_graph_one_fd_st' as a means of
avoiding an early-exit from non-library code.

However, 'load_commit_graph_one' does not support commit-graph chains,
and hence the 'read-graph' test tool does not work with them.

Replace 'load_commit_graph_one' with 'read_commit_graph_one' in order to
support commit-graph chains. In the spirit of 61df89c8e5,
'read_commit_graph_one' does not ever 'die()', making it a suitable
replacement here.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>