Implemet Cache for Stopwatches.APIFormat()

[6543]

extract from #12729 for v1.13.0 release

[6543]

loading issues and repos is now neede because of #12458 witch is in v1.13.0

this pull make it more efficient

[6543]

@techknowlogick hope you dont mind :)

[codecov-commenter]

Codecov Report

Merging #12730 into master will increase coverage by 0.03%.
The diff coverage is 84.00%.

@@            Coverage Diff             @@
##           master   #12730      +/-   ##
==========================================
+ Coverage   43.38%   43.41%   +0.03%     
==========================================
  Files         648      648              
  Lines       71548    71569      +21     
==========================================
+ Hits        31042    31073      +31     
+ Misses      35475    35468       -7     
+ Partials     5031     5028       -3     

Impacted Files	Coverage Δ
models/issue_stopwatch.go	73.33% <84.00%> (+2.79%)	⬆️
modules/util/timer.go	42.85% <0.00%> (-42.86%)	⬇️
modules/git/repo.go	49.74% <0.00%> (+0.50%)	⬆️
modules/log/event.go	59.43% <0.00%> (+1.88%)	⬆️
modules/process/manager.go	75.00% <0.00%> (+2.50%)	⬆️
modules/git/utils.go	77.04% <0.00%> (+3.27%)	⬆️
modules/indexer/stats/queue.go	76.47% <0.00%> (+11.76%)	⬆️
modules/indexer/stats/db.go	60.86% <0.00%> (+17.39%)	⬆️

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