Display pull request head branch even the branch deleted or repository deleted

[lunny]

This PR did some improvments.

• Merge loadBaseRepo/getBaseRepo, loadHeadRepo/getHeadRepo on pull request
• Only load once on loadHeadRepo if no system error
• Use issue.Repo if possible
• Display pull request head branch information even branch deleted or head repository deleted.
• Get sha if possilbe, from pull/x/head

Fix #10347

[ppmathis]

Thank you for implementing this feature request! As I would mainly need this functionality for Renovate as you correctly discovered, I would like to ask if this PR also exposes the head branch name through the API, specifically /api/v1/pulls and /api/v1/pulls/<PR>? Haven't tested it yet, but based on the diff view this currently looks like UI/view-only, is that correct?

[lunny]

@ppmathis Updated. OK, now it should include headbranch information on API also.

[6543]

pleace update tests ...

[codecov-io]

Codecov Report

Merging #10413 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10413      +/-   ##
==========================================
+ Coverage    43.7%   43.71%   +<.01%     
==========================================
  Files         585      585              
  Lines       82005    82005              
==========================================
+ Hits        35844    35848       +4     
+ Misses      41719    41716       -3     
+ Partials     4442     4441       -1

Impacted Files	Coverage Δ
services/pull/patch.go	61.93% <0%> (-2.59%)	⬇️
services/pull/temp_repo.go	29.05% <0%> (-2.57%)	⬇️
services/pull/check.go	50% <0%> (-2.44%)	⬇️
modules/queue/workerpool.go	56.93% <0%> (-2.14%)	⬇️
routers/repo/view.go	36.82% <0%> (+0.67%)	⬆️
models/notification.go	64.71% <0%> (+0.83%)	⬆️
modules/log/event.go	65.64% <0%> (+1.02%)	⬆️
modules/indexer/issues/indexer.go	57.52% <0%> (+1.32%)	⬆️
modules/queue/unique_queue_disk_channel.go	55.76% <0%> (+1.92%)	⬆️
models/unit.go	39.5% <0%> (+2.46%)	⬆️
... and 1 more

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[zeripath]

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