t/lib-commit-graph.sh: allow `graph_read_expect()` in sub-directories

t/lib-commit-graph.sh: allow `graph_read_expect()` in sub-directories

The `graph_read_expect()` function is used to ensure that the output of
the "read-graph" test helper matches certain parameters (e.g., how many
commits are in the graph, which chunks were written, etc.).

It expects the Git repository being tested to be at the current working
directory. However, a handful of t5318 tests use different repositories
stored in sub-directories. To work around this, several tests in t5318
change into the relevant repository outside of a sub-shell, altering the
context for the rest of the suite.

Prepare to remove these globally-scoped directory changes by teaching
`graph_read_expect()` to take an optional "-C dir" to specify where the
repository containing the commit-graph being tested is.

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