Fix PR merge error

[ethantkoenig]

Fix a bug where we used to look for commits between the merge commit and base commit in the head repo (where the merge commit does not exist). This led to spurious error messages (e.g. see line 1069 of https://drone.gitea.io/go-gitea/gitea/3392/9)

[codecov-io]

Codecov Report

Merging #3421 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3421      +/-   ##
==========================================
+ Coverage   35.58%   35.61%   +0.02%     
==========================================
  Files         281      281              
  Lines       40593    40591       -2     
==========================================
+ Hits        14445    14455      +10     
+ Misses      24008    23998      -10     
+ Partials     2140     2138       -2

Impacted Files	Coverage Δ
models/pull.go	52.16% <100%> (+0.12%)	⬆️
models/repo_list.go	65.62% <0%> (-1.57%)	⬇️
models/repo.go	42.98% <0%> (-0.19%)	⬇️
models/repo_indexer.go	46.01% <0%> (+3.98%)	⬆️
modules/process/manager.go	81.15% <0%> (+4.34%)	⬆️
modules/indexer/indexer.go	77.5% <0%> (+7.5%)	⬆️

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