t/test-repository: fix leaking repository

The test-repository test helper zeroes out `the_repository` such that it
can be sure that our codebase only ends up using the supplied repository
that we initialize in the respective helper functions. This does cause
memory leaks though as the data that `the_repository` has been holding
onto is not referenced anymore.

Fix this by calling `repo_clear()` instead.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5318-commit-graph.sh

@@ -1,6 +1,8 @@
 #!/bin/sh
 
 test_description='commit graph'
+
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 . "$TEST_DIRECTORY"/lib-chunk.sh