Add serialVersionUID declarations to 3 classes

Fix findbugs warning, otherwise harmless change
Declare JGitAPIImpl provider as transient, not serializable
Adapt some tests to repo path containing a space character

Other tests are not yet able to execute successfully if the path to the
working directory includes a space.  Since the Windows installer places
JENKINS_HOME in a directory path containing a space, those tests will
continue to fail.
Merge remote-tracking branch 'upstream/master' into master-use-explicit-encoding