Fix panic in API pulls when headbranch does not exist

[zeripath]

When a branch of a forked repository is merged as a PR and then deleted /api/v1/repos/owner/name/pulls and /api/v1/repos/owner/name/pulls/index will cause a 500 with a panic internally.

This is because:

• modules/git/repo_commit.go:21:GetRefCommitID(name string) does not return a git.ErrNotExist error
• modules/convert/pull.go:110:log.Error("GetBranch[%s]: %v", headBranch.Name, err) causes a NPE because headBranch is nil as determined a few lines earlier.

These are now fixed.

Fix #10639

Signed-off-by: Andrew Thornton [email protected]

[zeripath]

I'm fairly certain that 1.11 will be affected too.

[codecov-io]

Codecov Report

Merging #10676 into master will decrease coverage by 0.03%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master   #10676      +/-   ##
==========================================
- Coverage   43.59%   43.55%   -0.04%     
==========================================
  Files         589      589              
  Lines       82561    82565       +4     
==========================================
- Hits        35996    35965      -31     
- Misses      42104    42146      +42     
+ Partials     4461     4454       -7

Impacted Files	Coverage Δ
routers/repo/branch.go	55.72% <0%> (ø)	⬆️
modules/convert/pull.go	68.69% <0%> (-3.48%)	⬇️
modules/git/repo_commit.go	53.67% <100%> (+0.27%)	⬆️
modules/context/panic.go	50% <0%> (-50%)	⬇️
services/pull/check.go	38.41% <0%> (-15.86%)	⬇️
models/repo_mirror.go	2.12% <0%> (-10.64%)	⬇️
modules/queue/unique_queue_disk_channel.go	53.84% <0%> (-1.93%)	⬇️
services/pull/pull.go	34.7% <0%> (-1.18%)	⬇️
services/mirror/mirror.go	18.75% <0%> (-0.66%)	⬇️
models/pull.go	42.3% <0%> (-0.56%)	⬇️
... and 5 more

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