Add migration to fix the old broken merge-bases

[zeripath]

#10586 fixed the merge-bases for current PRs but did not fix the merge-base for merged PRs.

This PR adds a new migration that will run through all PRs in the system and recalculates the merge-bases.

[codecov-io]

Codecov Report

Merging #10604 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master   #10604      +/-   ##
==========================================
- Coverage    43.7%   43.69%   -0.02%     
==========================================
  Files         585      586       +1     
  Lines       82031    82098      +67     
==========================================
+ Hits        35852    35870      +18     
- Misses      41737    41788      +51     
+ Partials     4442     4440       -2

Impacted Files	Coverage Δ
models/migrations/migrations.go	4.16% <ø> (ø)	⬆️
models/migrations/v128.go	0% <0%> (ø)
modules/indexer/stats/queue.go	62.5% <0%> (-18.75%)	⬇️
modules/indexer/stats/db.go	40.62% <0%> (-9.38%)	⬇️
modules/git/utils.go	65.67% <0%> (-4.48%)	⬇️
modules/queue/workerpool.go	56.93% <0%> (-1.07%)	⬇️
modules/git/repo.go	46.78% <0%> (-0.92%)	⬇️
models/notification.go	64.71% <0%> (+0.83%)	⬆️
modules/log/event.go	65.64% <0%> (+1.02%)	⬆️
... and 6 more

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[lafriks]

make LG-TM work