Refactor: move Commit To APIFormat Code & Lot of StopWatch related things

[6543]

close #12723

StopWatch

• extend TestAPIListStopWatches
• remove type Stopwatches

Commit:

• create own file for related things in convert package
• rename ToCommit ToPayloadCommit
• move toCommit into convert package

[codecov-commenter]

Codecov Report

Merging #12729 into master will decrease coverage by 0.10%.
The diff coverage is 87.91%.

@@            Coverage Diff             @@
##           master   #12729      +/-   ##
==========================================
- Coverage   43.09%   42.99%   -0.11%     
==========================================
  Files         655      656       +1     
  Lines       72347    72334      -13     
==========================================
- Hits        31181    31099      -82     
- Misses      36115    36207      +92     
+ Partials     5051     5028      -23     

Impacted Files	Coverage Δ
modules/convert/convert.go	74.58% <50.00%> (-2.04%)	⬇️
modules/convert/issue.go	87.30% <86.20%> (-0.33%)	⬇️
modules/convert/git_commit.go	88.28% <88.28%> (ø)
models/issue_stopwatch.go	70.37% <100.00%> (-2.97%)	⬇️
routers/api/v1/repo/commits.go	57.31% <100.00%> (-14.30%)	⬇️
routers/api/v1/repo/hook.go	26.41% <100.00%> (ø)
routers/api/v1/repo/issue_stopwatch.go	52.63% <100.00%> (ø)
modules/util/timer.go	42.85% <0.00%> (-42.86%)	⬇️
modules/indexer/stats/db.go	52.17% <0.00%> (-8.70%)	⬇️
modules/notification/ui/ui.go	72.22% <0.00%> (-8.34%)	⬇️
... and 12 more

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[lunny]

Otherwise LGTM

[lunny]

And I think this could be moved to v1.13 once it's ready.