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

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>