t7004: replace wc -l with modern test helpers

t7004: replace wc -l with modern test helpers

Pipelines of the form "test $(git tag | wc -l) -eq 0" suppress git's
exit code. This means a crash or unexpected failure from git tag would
go undetected. Additionally, the use of $(...) creates a subshell for
each check, which adds unnecessary overhead.

Replace these patterns with test_must_be_empty and test_line_count.
These helpers check the output of git directly from a file, ensuring
git's exit code is captured properly via the preceding "&&" chain.
They also provide better diagnostics on failure by printing the
contents of the file when a check does not pass.

Signed-off-by: Siddharth Shrimali <r.siddharth.shrimali@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>