Refactor "Content" for file uploading (#25851)

Before: the concept "Content string" is used everywhere. It has some
problems:

1. Sometimes it means "base64 encoded content", sometimes it means "raw
binary content"
2. It doesn't work with large files, eg: uploading a 1G LFS file would
make Gitea process OOM

This PR does the refactoring: use "ContentReader" / "ContentBase64"
instead of "Content"

This PR is not breaking because the key in API JSON is still "content":
`` ContentBase64 string `json:"content"` ``

tests/integration/pull_update_test.go

@@ -6,6 +6,7 @@ package integration
 import (
 	"net/http"
 	"net/url"
+	"strings"
 	"testing"
 	"time"
 
@@ -104,9 +105,9 @@ func createOutdatedPR(t *testing.T, actor, forkOrg *user_model.User) *issues_mod
 	_, err = files_service.ChangeRepoFiles(git.DefaultContext, baseRepo, actor, &files_service.ChangeRepoFilesOptions{
 		Files: []*files_service.ChangeRepoFile{
 			{
-				Operation: "create",
-				TreePath:  "File_A",
-				Content:   "File A",
+				Operation:     "create",
+				TreePath:      "File_A",
+				ContentReader: strings.NewReader("File A"),
 			},
 		},
 		Message:   "Add File A",
@@ -131,9 +132,9 @@ func createOutdatedPR(t *testing.T, actor, forkOrg *user_model.User) *issues_mod
 	_, err = files_service.ChangeRepoFiles(git.DefaultContext, headRepo, actor, &files_service.ChangeRepoFilesOptions{
 		Files: []*files_service.ChangeRepoFile{
 			{
-				Operation: "create",
-				TreePath:  "File_B",
-				Content:   "File B",
+				Operation:     "create",
+				TreePath:      "File_B",
+				ContentReader: strings.NewReader("File B"),
 			},
 		},
 		Message:   "Add File on PR branch",