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/web/web.go

@@ -4,7 +4,6 @@
 package web
 
 import (
-	gocontext "context"
 	"net/http"
 	"strings"
 
@@ -1521,24 +1520,23 @@ func registerRoutes(m *web.Router) {
 
 		m.Group("/blob_excerpt", func() {
 			m.Get("/{sha}", repo.SetEditorconfigIfExists, repo.SetDiffViewStyle, repo.ExcerptBlob)
-		}, func(ctx *context.Context) gocontext.CancelFunc {
+		}, func(ctx *context.Context) {
 			// FIXME: refactor this function, use separate routes for wiki/code
 			if ctx.FormBool("wiki") {
 				ctx.Data["PageIsWiki"] = true
 				repo.MustEnableWiki(ctx)
-				return nil
+				return
 			}
 
 			if ctx.Written() {
-				return nil
+				return
 			}
-			cancel := context.RepoRef()(ctx)
+			context.RepoRef()(ctx)
 			if ctx.Written() {
-				return cancel
+				return
 			}
 
 			repo.MustBeNotEmpty(ctx)
-			return cancel
 		})
 
 		m.Group("/media", func() {