t5323: drop mentions of "master"

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>