t5318: avoid top-level directory changes

Avoid changing the current working directory from outside of a sub-shell
during the tests in t5318.

Each test has mostly straightforward changes, either:

  - Removing the top-level `cd "$TRASH_DIRECTORY/full"`, which is
    unnecessary after ensuring that other tests don't change their
    working directory outside of a sub-shell.

  - Changing any Git invocations which want to be in a sub-directory by
    either (a) adding a "-C $DIR" argument, or (b) moving the whole test
    into a sub-shell.

While we're here, remove any explicit "git config core.commitGraph true"
invocations which were designed to enable use of the commit-graph. These
are unnecessary following 31b1de6 (commit-graph: turn on
commit-graph by default, 2019-08-13).

Signed-off-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ttaylorr