Various Merge Base fixes

[zeripath]

• Allow PRs with deleted base branches to still show diff
• Fix broken merge base migration in v128 for merged PR. Related to [Bug] Merged PRs have their commits and files changed metadata 'zeroed'... sometimes. #10766

[codecov-io]

Codecov Report

Merging #10786 into master will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff            @@
##           master   #10786    +/-   ##
========================================
  Coverage   43.45%   43.46%            
========================================
  Files         592      593     +1     
  Lines       82905    83005   +100     
========================================
+ Hits        36027    36074    +47     
- Misses      42402    42451    +49     
- Partials     4476     4480     +4     

Impacted Files	Coverage Δ
models/migrations/migrations.go	4.16% <ø> (ø)
models/migrations/v128.go	0.00% <0.00%> (ø)
models/migrations/v134.go	0.00% <0.00%> (ø)
routers/repo/pull.go	28.01% <0.00%> (-0.82%)	⬇️
models/unit.go	41.97% <0.00%> (-2.47%)	⬇️
models/pull.go	42.85% <0.00%> (+0.55%)	⬆️
services/pull/pull.go	35.88% <0.00%> (+0.88%)	⬆️
services/pull/patch.go	64.51% <0.00%> (+2.58%)	⬆️
services/pull/temp_repo.go	34.18% <0.00%> (+5.12%)	⬆️
... and 1 more

---

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[lunny]

It's better to add some tests.

[zeripath]

Tests? Where can we put them? I'd love it if we had tests for migrations but we don't have infrastructure for them. I'm happy to begin doing that but it's going to be a huge PR.

[lunny]

We can add a function on modules/git named CalculateMergebase And add some tests on that package.

[zeripath]

This function should only be used in the migration otherwise we should simply be storing the actual MergeBase used for PRs. There's no reason to have to recalculate it.

Ideally this fixed migration would only run on PRs that were merged before v128 was run.

[zeripath]

@lunny needs your review

[lafriks]

Please resolve conflicts

[zeripath]

conflicts resolved.

[lafriks]

not anymore :P

[zeripath]

eurgh we need a better way of doing migrations!

[zeripath]

conflicts resolved again ... 😛

[lafriks]

@lunny need your approval

Just don't block this change.

[lafriks]

Just tested migration v134.go on copy of my production server and it does fix broken PR's that I had

[lafriks]

Please send backport