Independent maintenance requests from the same target: broken state after accepting one

[lrupp]

Issue Description

I have two independent maintenance requests (nagios and ansible) for the same target: Leap 15.1. Once the first request gets accepted, the other one gets into a broken state, as the complete repository gets wiped.

Expected Result

• OBS should only delete the package mentioned in a maintenance request
• finally, OBS should check for an empty repository and ONLY delete this repository if the repository is really empty

How to Reproduce

1. Go to https://build.opensuse.org/request/show/791861
2. Check the two incidents:
   • https://build.opensuse.org/request/show/791861#Incident_nagios_openSUSE_Leap_15_1_Update
   • https://build.opensuse.org/request/show/791861#Incident_ansible_openSUSE_Leap_15_1_Update

The first one is broken: "The diff call for home:lrupp:branches:openSUSE:Leap:15.1:Update/nagios.openSUSE_Leap_15.1_Update failed: package 'nagios.openSUSE_Leap_15.1_Update' does not exist"

The second one is now revoked in the hope that this keeps the status of Request 791861 as it is to show it in this bug.

Detailed reproducer:

1. osc branch -M -c openSUSE:Leap:15.1 ansible
2. osc branch -M -c openSUSE:Leap:15.1 nagios
3. <do changes, submit>
4. inside the branch for the ansible package: osc mr
5. inside the branch for the nagios pacakge: osc mr
6. accept one of the created maintenance requests

[hennevogel]

Similar to #9241