Don't run nightly main branch tasks when they have not changed

[sbidoul]

Is your feature request related to a problem?

Over time, with the number of repos and branches growing, the main branch bot is taking longer and longer.
Since it fills the queue, some more important commands wait for a long time before being executed.

Describe the solution you'd like

An "easy" optimization would be to cache the last commit processed and not re-run the main branch task if the head of the branch has not changed.

Describe alternatives you've considered

• do nothing: not sustainable in the long run
• investigate running main branch task with a lower priority (in the sense of the celery queues) to free workers for high priority tasks such as merge commands

Additional context

Related to #235

[sbidoul]

Using redis to store the commits of the branches we have processed would be ideal.

[pedrobaeza]

This morning the commands have been processed before. I thought you did something already...

[sbidoul]

This night weblate has been quiet with a much lower load on the machine, that's why the nightly bot jobs finished earlier.