it happended to me, too. The setupWizzard step, it showed Offline.
I searched the launch log and listed below
2016-09-19T08:30:45.649221697Z Sep 19, 2016 8:30:45 AM hudson.model.AsyncPeriodicWork$1 run
2016-09-19T08:30:45.649488934Z INFO: Finished Download metadata. 3,908 ms
2016-09-19T08:30:46.037168472Z [33mSep 19, 2016 8:30:46 AM hudson.model.UpdateCenter updateDefaultSite
2016-09-19T08:30:46.037231577Z WARNING: Upgrading Jenkins. Failed to update the default Update Site 'default'. Plugin upgrades may fail.
2016-09-19T08:30:46.037255668Z java.io.IOException: Could not find JSON in http://updates.jenkins-ci.org/update-center.json?id=default&version=2.7.4
2016-09-19T08:30:46.037320350Z at hudson.model.DownloadService.loadJSON(DownloadService.java:174)
2016-09-19T08:30:46.037354565Z at hudson.model.UpdateSite.updateDirectlyNow(UpdateSite.java:176)
2016-09-19T08:30:46.037684341Z at hudson.model.UpdateCenter.updateDefaultSite(UpdateCenter.java:2181)
2016-09-19T08:30:46.037725077Z at jenkins.install.SetupWizard.init(SetupWizard.java:170)
2016-09-19T08:30:46.037763994Z at jenkins.install.InstallState$3.initializeState(InstallState.java:105)
2016-09-19T08:30:46.037791411Z at jenkins.model.Jenkins.setInstallState(Jenkins.java:970)
2016-09-19T08:30:46.037842570Z at jenkins.install.InstallUtil.proceedToNextStateFrom(InstallUtil.java:94)
2016-09-19T08:30:46.037872111Z at jenkins.model.Jenkins.<init>(Jenkins.java:887)
2016-09-19T08:30:46.037911774Z at hudson.model.Hudson.<init>(Hudson.java:85)
2016-09-19T08:30:46.037939394Z at hudson.model.Hudson.<init>(Hudson.java:81)
2016-09-19T08:30:46.037965732Z at hudson.WebAppMain$3.run(WebAppMain.java:227)
Please provide more details of what you were doing when that happened, if you found any work around to the issue, and especially why you think that is an issue with the git plugin.
I've assumed this is unrelated to the git plugin, so have assigned it to core. I suspect someone on core will assign it to infrastructure, since that seems the most likely area.