See here and here
GitPluginTest is currently failing in the ATH since build number 72
Complete the PCT flow started in JENKINS-50540 for git plugin
Slow performance of /pluginManager/available rendering
Not sure if the fix in JENKINS-51205 is enough, I am still receiving email alerts about instability on git-plugin, however at this moment ci.jenkins.io is down so I can not check the status
It seems it worked, I am going to get this open for a pair of days and close if nothing new arises
Last builds of git-plugin have no failures, so I am going to close this.
The fix for JENKINS-51205 solved this problem