fix: Correctly verify signatures when targetRevision is a branch name (cherry-pick #14214)

[jannfis]

Cherry-picks #14214 into release-2.7

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[codecov]

Codecov Report

Patch coverage: 88.00% and project coverage change: +0.01 🎉

Comparison is base (3424fa4) 48.89% compared to head (61e68b6) 48.91%.

Additional details and impacted files

@@               Coverage Diff               @@
##           release-2.7   #14236      +/-   ##
===============================================
+ Coverage        48.89%   48.91%   +0.01%     
===============================================
  Files              247      247              
  Lines            42762    42781      +19     
===============================================
+ Hits             20909    20925      +16     
- Misses           19741    19742       +1     
- Partials          2112     2114       +2     

Impacted Files	Coverage Δ
reposerver/repository/repository.go	60.50% <70.00%> (+0.03%)	⬆️
util/exec/exec.go	100.00% <100.00%> (ø)
util/git/client.go	51.08% <100.00%> (+1.08%)	⬆️

... and 1 file with indirect coverage changes

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