t5500: simplify test implementation and fix git exit code suppression

t5500: simplify test implementation and fix git exit code suppression

The 'shallow since with commit graph and already-seen commit”
test uses a convoluted here-doc that combines manual input
construction with packetize, echo and embedded Git commands.
This structure hides failures from the git commands,
as their exit codes are suppressed inside echo command
substitution and being on the upstream side of pipes.

Instead of using here-doc to construct the pack
protocol that is directly sent to the
'git upload-pack' command being tested,
capture the outputs of the git commands upfront
and use the 'test-tool pkt-line pack'
tool to construct the input in a temporary file,
and then feed it to the command.

This has a few advantages:

* Executing the git commands outside the here-doc
avoids suppressing their exit codes and makes
debugging easier.

* It removes the need to manually count and
manage pkt-line lengths to keep in line with
the v2 protocol, as the tool handles this internally.

Signed-off-by: Shreyansh Paliwal <shreyanshpaliwalcmsmn@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>