Signed-off-by: Paulo Gomes <paulo.gomes@weave.works>

ForceGoGitImplementation ignores the value set for gitImplementation
and ensures that go-git is used for all GitRepository objects.
This can be used to confirm that Flux instances won't break if/when
the libgit2 implementation was to be deprecated.

When enabled, libgit2 won't be initialized, nor will any git2go cgo
code be called.

Signed-off-by: Paulo Gomes <paulo.gomes@weave.works>

Signed-off-by: Paulo Gomes <paulo.gomes@weave.works>