fix branch exists conditional

[motatoes]

We have a check for branch exists here

digger/backend/controllers/github.go

Line 463 in f2e3f8d

if !branchExists {

but it seems that it still returns false even when branch autodelete is turned off. We need to fix that conditional and figure out why it is returning true even with github autodelete

[motatoes]

Turns out this was a race condition and it happens periodically, for now I fixed it with a time.sleep() here https://github.com/diggerhq/digger/blob/develop/backend/controllers/github.go#L450