lib-log-graph: consolidate test_cmp_graph logic

lib-log-graph: consolidate test_cmp_graph logic

Log graph comparision logic is duplicated many times in:

- t3430-rebase-merges.sh
- t4202-log.sh
- t4214-log-graph-octopus.sh
- t4215-log-skewed-merges.sh

Consolidate the core of the comparision and sanitization logic in
lib-log-graph, and use it to replace the existing tests.

While at it, lose the singular/plural transition magic from the
sanitize_output helper, which was necessary around 7f814632 ("Use
correct grammar in diffstat summary line", 2012-02-01), that has
long outlived its usefulness.

Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>