[PATCH] tests: prefer host Git to verify chainlint self-checks

The "check-chainlint" target runs automatically when running tests and performs self-checks to verify that the chainlinter itself produces the expected output. Originally, the chainlinter was implemented via sed, but the infrastructure has been rewritten in fb41727b7e (t: retire unused chainlint.sed, 2022-09-01) to use a Perl script instead.

The rewrite caused some slight whitespace changes in the output that are ultimately not of much importance. In order to be able to assert that the actual chainlinter errors match our expectations we thus have to ignore whitespace characters when diffing them. As the `-w` flag is not in POSIX we try to use `git diff -w --no-index` before we fall back to `diff -w -u`.

To accomodate for cases where the host system has no Git installation we use the locally-compiled version of Git. This can result in problems though when the Git project's repository is using extensions that the locally-compiled version of Git doesn't understand. It will refuse to run and thus cause the checks to fail.

Fix this issue by prefering the host's Git resolved via PATH. If it doesn't exist, then we fall back to the locally-compiled Git version and diff as before.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
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I've started to dogfood the reftable backend on my local machine and have converted many repositories to use the reftable backend. This surfaced the described issue because the repository now sets up the "extensions.refStorage" extension, and thus "check-chainlint" fails depending on which versions of Git I'm trying to compile and test.

 t/Makefile | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)