t5323: drop mentions of "master"

Commit 0696232390 (pack-redundant: fix crash when one packfile in repo, 2020-12-16) added one some new tests to t5323. At the time, the sub-repo we used was called "master". But in a parallel branch, this was switched to "main". When the latter branch was merged in 27d7c8599b (Merge branch 'js/default-branch-name-tests-final-stretch', 2021-01-25), some of those spots caused textual conflicts, but some (for tests that were far enough away from other changed code) were just semantic. The merge resolution fixed up most spots, but missed this one. Even though this did impact actual code, it turned out not to fail the tests. Running 'cd "$master_repo"' ended up staying in the same directory, running the test in the main trash repo instead of the sub-repo. But because the point of the test is checking behavior when there are no packfiles, it worked in either repo (since both are empty at this point in the script). Reported-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>