Change 'osc rmkpac' to require fragment with branch name in --scmsync url

[dmach]

Fixes: #2116

[codecov]

Codecov Report

❌ Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 33.93%. Comparing base (5c240dc) to head (ef1615d).
⚠️ Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
osc/commands/rmkpac.py	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2127      +/-   ##
==========================================
- Coverage   33.94%   33.93%   -0.01%     
==========================================
  Files         199      199              
  Lines       27035    27039       +4     
==========================================
- Hits         9177     9176       -1     
- Misses      17858    17863       +5     

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