t5314: check exit code of "git"

t5314: check exit code of "git"

Amend the test added in [1] to check the exit code of the "git"
invocations. An in-flight change[2] introduced a memory leak in these
invocations, which went undetected unless we were running under
"GIT_TEST_SANITIZE_LEAK_LOG=true".

Note that the in-flight change made 8 test files fail, but as far as I
can tell only this one would have had its exit code hidden unless
under "GIT_TEST_SANITIZE_LEAK_LOG=true". The rest would be caught
without it.

We could pick other variable names here than "ln%d", e.g. "commit",
"dummy_blob" and "file_blob", but having the "rev-parse" invocations
aligned makes the difference between them more readable, so let's pick
"ln%d".

1. 4cf2143e029 (pack-objects: break delta cycles before delta-search
   phase, 2016-08-11)
2. https://lore.kernel.org/git/221128.868rjvmi3l.gmgdl@evledraar.gmail.com/
3. faececa53f9 (test-lib: have the "check" mode for SANITIZE=leak
   consider leak logs, 2022-07-28)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>