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_list.go

@@ -754,7 +754,7 @@ func Issues(ctx *context.Context) {
 		}
 		ctx.Data["Title"] = ctx.Tr("repo.issues")
 		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)
 	}
 
 	projectIDs := parseProjectIDsFromQuery(ctx)