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]

(From OE-Core rev: 66be32c1a075201d6ee0e9b9e10b84e6a2ace745)

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

scripts/oe-selftest

@@ -36,7 +36,6 @@ import re
 import fnmatch
 import collections
 import imp
-import git
 
 sys.path.insert(0, os.path.dirname(os.path.realpath(__file__)) + '/lib')
 import scriptpath
@@ -577,6 +576,7 @@ def main():
         log.info("Finished")
 
         if args.repository:
+            import git
             # Commit tests results to repository
             metadata = metadata_from_bb()
             git_dir = os.path.join(os.getcwd(), 'selftest')