t/lib-commit-graph.sh: avoid sub-shell in `graph_git_behavior()`

t/lib-commit-graph.sh: avoid sub-shell in `graph_git_behavior()`

In a previous commit, we introduced a sub-shell in the implementation of
`graph_git_behavior()`, in order to allow us to pass `-C "$DIR"`
directly to the git processes spawned by `graph_git_two_modes()`.

Now that its callers are always operating from the "$TRASH_DIRECTORY"
instead of one of its sub-directories, we can drop the inner sub-shell,
as it is no longer required.

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