oe-selftest: import git module only when needed

oe-selftest: import git module only when needed

git module is not included into standard Python
library and therefore causes import errors on the systems
where PythonGit is not installed.

As git module only used in the code implementing --repository
functionality it's better to import git only in the scope
that requires it.

[YOCTO #10821]

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>