test(3p/cgit): t0107: support older and/or non-GNU tar r/2897

The untar tests for various compression algorithms use shortcut options
from GNU tar to handle decompression.  These options may not be provided
by non-GNU tar nor even by slightly older GNU tar versions which ship on
many systems.

An example of the latter case is the --zstd option.  This was added in
GNU tar-1.32 (2019-02-23)¹.  This version of tar is not provided by
CentOS/RHEL, in particular.  In Debian, --zstd has been backported to
the tar-1.30 release.

Avoid the requirement on any specific implementations or versions of tar
by piping decompressed output to tar.  This is compatible with older GNU
tar releases as well as tar implementations from other vendors.  (It may
also be a slight benefit that this more closely matches what the
snapshot creation code does.)

¹ Technically, the --zstd option was first released in tar-1.31
  (2019-01-02), but this release was very short-lived and is no longer
  listed on the GNU Tar release page.

Signed-off-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Change-Id: Ib65412cd1d23312ddd4cf840c09efc32512d3122

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