tar: Make all gzip compression hermetic #75
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We weren't passing the flag to make gzip hermetic. Without the flag to
ignore timestamp, the shasum will differ based on the system time.
Also, DEFAULT_ARGS contained "--options=gzip:...", which is invalid when
using any other compression scheme. Therefore, remove that option from
DEFAULT_ARGS and provide an alternate function called
add_default_compression_args
to add per-compression type defaultarguments which can do things like make the compression more hermetic.
Repro instructions:
Before, the above test would fail with sha256sum mismatches. With this patch, all shasums match.