Handle deleted base branch in PR

[zeripath]

If you delete the base branch of a PR, although the PR will be closed if you view the PR you will be presented with a 500. This PR will handle this case.

See for an example PR https://try.gitea.io/arandomer/anudderrepo/pulls/1

Signed-off-by: Andrew Thornton art27@cantab.net

[guillep2k]

This seems to work OK on all the pages I could test. I didn't test deleting the ref from refs/pulls/# and gc/prunning because I don't think that case it supposed to be covered by this PR. 👍

[zeripath]

@guillep2k deleting the internal ref is most definitely not covered by this PR and probably should be an internal server error - it should not be deleted unless you actually delete the PR.

[codecov-io]

Codecov Report

Merging #10618 into master will increase coverage by <.01%.
The diff coverage is 11.11%.

@@            Coverage Diff             @@
##           master   #10618      +/-   ##
==========================================
+ Coverage   43.69%   43.69%   +<.01%     
==========================================
  Files         587      587              
  Lines       82174    82183       +9     
==========================================
+ Hits        35905    35910       +5     
- Misses      41819    41824       +5     
+ Partials     4450     4449       -1

Impacted Files	Coverage Δ
routers/repo/pull.go	28.89% <11.11%> (-0.17%)	⬇️
services/pull/check.go	50% <0%> (-5.49%)	⬇️
modules/git/command.go	86.95% <0%> (-2.61%)	⬇️
services/pull/patch.go	61.93% <0%> (-2.59%)	⬇️
services/pull/temp_repo.go	29.05% <0%> (-2.57%)	⬇️
modules/indexer/issues/indexer.go	56.19% <0%> (-1.33%)	⬇️
models/gpg_key.go	53.95% <0%> (+0.52%)	⬆️
modules/queue/workerpool.go	59.07% <0%> (+1.06%)	⬆️
modules/queue/unique_queue_disk_channel.go	55.76% <0%> (+1.92%)	⬆️
modules/git/repo.go	49.08% <0%> (+2.29%)	⬆️
... and 3 more

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