Refactor request context (#32956)

Introduce RequestContext: is a short-lived context that is used to store
request-specific data.

RequestContext could be used to clean form tmp files, close context git
repo, and do some tracing in the future.

Then a lot of legacy code could be removed or improved. For example:
most `ctx.Repo.GitRepo.Close()` could be removed because the git repo
could be closed when the request is done.

routers/api/v1/repo/file.go

@@ -287,13 +287,12 @@ func GetArchive(ctx *context.APIContext) {
 	//     "$ref": "#/responses/notFound"
 
 	if ctx.Repo.GitRepo == nil {
-		gitRepo, err := gitrepo.OpenRepository(ctx, ctx.Repo.Repository)
+		var err error
+		ctx.Repo.GitRepo, err = gitrepo.RepositoryFromRequestContextOrOpen(ctx, ctx, ctx.Repo.Repository)
 		if err != nil {
 			ctx.Error(http.StatusInternalServerError, "OpenRepository", err)
 			return
 		}
-		ctx.Repo.GitRepo = gitRepo
-		defer gitRepo.Close()
 	}
 
 	archiveDownload(ctx)