Fix NPE on try to get tag reference via API

[6543]

In current situation, you need to add the gitRepo into tag, but this is not always posible as it's an unexported field. so if you try to get a tag via api it will cause a 500. this fix it.

fix:

2022/01/12 00:13:46 ...common/middleware.go:70:1() [E] PANIC: runtime error: invalid memory address or nil pointer dereference
	/usr/lib/go/src/runtime/panic.go:221 (0x44dca6)
		panicmem: panic(memoryError)
	/usr/lib/go/src/runtime/signal_unix.go:735 (0x44dc76)
		sigpanic: panicmem()
	/home/user/gitea/modules/git/repo_commit_nogogit.go:71 (0xac4ed1)
		(*Repository).getCommit: wr, rd, cancel := repo.CatFileBatch(repo.Ctx)
	/home/user/gitea/modules/git/tag.go:30 (0xad384d)
		(*Tag).Commit: return tag.repo.getCommit(tag.Object)
	/home/user/gitea/modules/git/repo_tree_nogogit.go:37 (0xad381c)
		(*Repository).getTree: commit, err := tag.Commit()
	/home/user/gitea/modules/git/repo_tree_nogogit.go:85 (0xad3f44)
		(*Repository).GetTree: return repo.getTree(id)
	/home/user/gitea/services/repository/files/tree.go:25 (0x1b88257)
		GetTreeBySHA: gitTree, err := gitRepo.GetTree(sha)
	/home/user/gitea/routers/api/v1/repo/tree.go:63 (0x1c66b78)
		GetTree: if tree, err := files_service.GetTreeBySHA(ctx.Repo.Repository, sha, ctx.FormInt("page"), ctx.FormInt("per_page"), ctx.FormBool("recursive")); err != nil {
	/home/user/gitea/modules/web/route.go:80 (0x1b5ff26)
		Wrap.func1: t(ctx)
	/usr/lib/go/src/net/http/server.go:2047 (0x77948e)
		HandlerFunc.ServeHTTP: f(w, r)
	/home/user/gitea/modules/context/api.go:436 (0x13805af)
		RepoRefForAPI.func1: next.ServeHTTP(w, req)
	/usr/lib/go/src/net/http/server.go:2047 (0x77948e)
		HandlerFunc.ServeHTTP: f(w, r)
	/home/user/gitea/modules/web/route.go:95 (0x1b600a5)

[codecov-commenter]

Codecov Report

Merging #18245 (7c4934c) into main (67d7388) will increase coverage by 0.01%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##             main   #18245      +/-   ##
==========================================
+ Coverage   45.63%   45.65%   +0.01%     
==========================================
  Files         830      830              
  Lines       92024    92021       -3     
==========================================
+ Hits        41993    42010      +17     
+ Misses      43289    43270      -19     
+ Partials     6742     6741       -1     

Impacted Files	Coverage Δ
modules/git/repo_tag.go	61.60% <ø> (-0.61%)	⬇️
modules/git/repo_tree_nogogit.go	49.01% <0.00%> (ø)
modules/git/repo_commit_nogogit.go	59.34% <100.00%> (-0.45%)	⬇️
modules/git/tag.go	84.31% <100.00%> (ø)
modules/repository/repo.go	47.84% <100.00%> (ø)
routers/api/v1/repo/tag.go	56.25% <100.00%> (ø)
services/repository/push.go	53.05% <100.00%> (ø)
modules/queue/workerpool.go	50.00% <0.00%> (-0.77%)	⬇️
models/repo_list.go	76.62% <0.00%> (-0.60%)	⬇️
models/issue_comment.go	54.51% <0.00%> (+0.28%)	⬆️
... and 5 more

---

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[lunny]

How about remove the repo field from tag?

[6543]

done