Confirm that JGit 3.4.1 [Fixed JENKINS-23424]

Prior to JGit 3.4.1, tracked files which were locally modified were
not reset to their unmodified state by JGit checkout, while the CliGit
arguments used in the same cases would reset the locally modified file
to their unmodified state.
Add test of credentials item clear method
Ignore Netbeans local configuration file
Merge remote-tracking branch 'origin/master' into master-add-Alexander-Link-fixes