Use @NonNull annotation of getRepository method

Better describes the contract between getRepository and its callers,
and reduces findbugs output by declaring that contract.
Check return value of File.delete, log warning if file not deleted

Logging a warning if a file is not deleted seems safe and will help
locate cases where files are not being properly removed.
Adjust the suppression of format string uses newline to correct class
Make TIMEOUT and USE_CLI final since they are values of properties
Declare CliGitAPIImpl.launcher transient in case class or subclass is serialized
Remove uncalled private method getGitCredentialsURL(String, String)