gittest: Do not run commands in seed Git repositories directly

While we already ensure that no seed test Git repositories are being
written into by running tests under a different user in our CI, we don't
check whether the repositories are used to spawn any read-only commands.
And this has in fact been fine until now: permissions were such that
this always worked.

This has changed with CVE-2022-24765 though: Git has started to operate
in repositories completely in case it is owned by a different user. As a
consequence, this breaks our testing infrastructure whenever a test is
trying to run a command in the test seed repositories directly given
that they're owned by `root`, and we ourselves run with UID 9999 in our
CI.

Luckily, commands like git-upload-pack(1) still work, which means that
we're still able to clone such repositories. And there are only very few
tests which don't first use e.g. `gittest.CloneRepository()` before
actually run commands directly in there, too, because we don't expose a
simple way to obtain the path of those seed repositories outside of the
`gittest` package. The only offenders are `gittest.ChecksumTestRepo()`
and `gittest.BundleTestRepo()`.

Refactor those two helper functions to instead operate on a cloned
repository instead of running commands in the seed repositories
directly. This fixes compatibility with all Git versions which include a
fix for above CVE.

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