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.
JENKINS-25353: Cleaned up index.lock file when checkout is aborted
Test index.lock removal - JENKINS-25353
Make JGitAPIImpl default behavior match CliGitAPIImpl default

Previously, if JGitAPIImpl was constructed with a null workspace, it
would later throw exceptions. If CliGitAPIImpl was constructed with a null
workspace, it would use the current working directory as the workspace.

Since CliGitAPIImpl is the reference implementation, make JGitAPIImpl
match it.
Update bugs link in README

Projects were renamed in Jira
Note that javadoc improvements are in progress
Merge branch 'master' into master_ialbors

Conflicts:
	src/main/java/org/jenkinsci/plugins/gitclient/JGitAPIImpl.java