t4014: use test_line_count() where possible

t4014: use test_line_count() where possible

Convert all instances of `cnt=$(... | wc -l) && test $cnt = N` into uses
of `test_line_count()`.

While we're at it, convert one instance of a Git command upstream of a
pipe into two commands. This prevents a failure of a Git command from
being masked since only the return code of the last member of the pipe
is shown.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>