Fix 'linkdiff' command error 400: prj/pac/md5 not in repository

[dmach]

Fixes: #1468
Fixes: #1437

[codecov]

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Comparison is base (116be57) 31.25% compared to head (472a56f) 31.24%.
Report is 14 commits behind head on master.

Files	Patch %	Lines
osc/commandline.py	0.00%	15 Missing ⚠️
osc/git_scm/store.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1470      +/-   ##
==========================================
- Coverage   31.25%   31.24%   -0.02%     
==========================================
  Files          49       49              
  Lines       17820    17826       +6     
==========================================
  Hits         5570     5570              
- Misses      12250    12256       +6     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[hramrach]

Now linkdiff of branched packages also works 🥇