Prevent double-close of issues

[zeripath]

It's currently possible to end double-closing issues thus leading to a negative number of pulls remaining to be closed. This PR reloads the issue from the database within the transaction and checks that it isn't already closed in the database.

[codecov-io]

Codecov Report

Merging #6233 into master will decrease coverage by 0.01%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master    #6233      +/-   ##
==========================================
- Coverage   38.84%   38.83%   -0.02%     
==========================================
  Files         355      355              
  Lines       50239    50243       +4     
==========================================
- Hits        19516    19512       -4     
- Misses      27897    27903       +6     
- Partials     2826     2828       +2

Impacted Files	Coverage Δ
models/issue.go	48.26% <33.33%> (-0.08%)	⬇️
modules/process/manager.go	76.81% <0%> (-4.35%)	⬇️
models/repo_list.go	66.84% <0%> (-1.06%)	⬇️

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