[Bug] GetDiverging(pull) can lock pull in a 500 state

[6543]

if zwo threads add remote repo to pullBase repo they can lock it

2020/02/03 02:47:28 routers/repo/pull.go:392:PrepareViewPullInfo() [E] GetDiverging: headGitRepo.AddRemote: exit status 128 - fatal: remote tmp-pull-4206-base already exists.
https://gitea.com/gitea/go-sdk/pulls/256

[6543]

solution: use a tempRepo to compare base & head repo ...

-> but to save resorces we need a cahcke or precalc the values on each push to pull

move this "task" to checkPullTask?