t5516: fix test order flakiness

t5516: fix test order flakiness

The 'fetch follows tags by default' test sorts using 'sort -k 4', but
for-each-ref output only has 3 columns. This relies on sort treating records
with fewer fields as having an empty fourth field, which may produce
unstable results depending on locale. This appears to be an accident added
in 3f763ddf28 (fetch: set remote/HEAD if it does not exist, 2024-11-22).

Use 'sort -k 3' to match the actual number of columns in the output.

Reviewed-by: Matthew John Cheetham <mjcheetham@outlook.com>
Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>