t5318: don't pass non-object directory to '--object-dir'

In f237c8b (commit-graph: implement git-commit-graph write,
2018-04-02) the test t5318.3 was introduced to ensure that calling 'git
commit-graph write' in a repository with no packfiles does not write any
commit-graph file(s).

To exercise more paths in 'builtin/commit-graph.c', this test passes
'--object-dir' to 'git commit-graph write', but the given argument
refers to the working copy, not the object directory.

Since the commit-graph sub-commands currently swallow these errors, this
does not result in a test failure. But, it is only lucky that the test
ends with no commit-graphs, since there were none to begin with.

In preparation for a future commit where an '--object-dir' argument that
does not match a known object directory will print out a failure, let's
fix the test to still use '--object-dir', but pass the correct location
to the object store instead of '.'.

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

t/t5318-commit-graph.sh

@@ -19,8 +19,8 @@ test_expect_success 'verify graph with no graph file' '
 
 test_expect_success 'write graph with no packs' '
 	cd "$TRASH_DIRECTORY/full" &&
-	git commit-graph write --object-dir . &&
-	test_path_is_missing info/commit-graph
+	git commit-graph write --object-dir $objdir &&
+	test_path_is_missing $objdir/info/commit-graph
 '
 
 test_expect_success 'exit with correct error on bad input to --stdin-packs' '