Most time, when invoking `git.OpenRepository`, `objectFormat` will not
be used, so it's a waste to invoke commandline to get the object format.
This PR make it a lazy operation, only invoke that when necessary.
(cherry picked from commit e84e5db6de0306d514b1f1a9657931fb7197a188)
The test suite was broken e.g. on Debian 12 due to requiring a very
recent version of Git installed on the system. This commit skips SHA256
tests in the git module, if a Git version older than 2.42 or gogit is used.
