t/lib-commit-graph.sh: avoid directory change in `graph_git_behavior()`

The `graph_git_behavior()` helper asserts that a number of common Git operations (such as `git log --oneline`, `git log --topo-order`, etc.) produce identical output regardless of whether or not a commit-graph is in use. This helper takes as its second argument the location (relative to the `$TRASH_DIRECTORY`) of the Git repostiory under test. In order to run each of its commands within that repository, it first changes into that directory, without the use of a sub-shell. This pollutes future tests which expect to be run in the top-level `$TRASH_DIRECTORY` as usual. We could wrap `graph_git_behavior()` in a sub-shell, like: graph_git_behavior() { # ... ( cd "$TRASH_DIRECTORY/$DIR" && graph_git_two_modesl ) } , but since we're invoking git directly, we can pass along a "-C $DIR" when "$DIR" is non-empty. Note, however, that until the remaining callers are cleaned up to avoid changing working directories outside of a sub-shell, that we need to ensure that we are operating in the top-level $TRASH_DIRECTORY. The inner-subshell will go away in a future commit once it is no longer necessary. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>