refactor: decouple git.Repository(ctx) from git.Commit & git.Tree (#38464)

1. Storing "ctx" in a long-living object is wrong
2. Make the commit & tree cacheable (for the future performance
optimization)
3. Also fix some bad designs like `// FIXME: bad design, this field can
be nil if the commit is from "last commit cache"`

ref:
* #33893

routers/web/repo/issue_view.go

@@ -334,7 +334,7 @@ func ViewIssue(ctx *context.Context) {
 			return
 		}
 		ctx.Data["PageIsIssueList"] = true
-		ctx.Data["NewIssueChooseTemplate"] = issue_service.HasTemplatesOrContactLinks(ctx.Repo.Repository, ctx.Repo.GitRepo)
+		ctx.Data["NewIssueChooseTemplate"] = issue_service.HasTemplatesOrContactLinks(ctx, ctx.Repo.Repository, ctx.Repo.GitRepo)
 	}
 
 	ctx.Data["IsProjectsEnabled"] = ctx.Repo.Permission.CanRead(unit.TypeProjects)