Init git module before database migration (#21764)

Close #21761

Some database migrations depend on the git module.

models/migrations/migrations.go

@@ -6,6 +6,7 @@
 package migrations
 
 import (
+	"context"
 	"fmt"
 	"os"
 
@@ -23,6 +24,7 @@ import (
 	"code.gitea.io/gitea/models/migrations/v1_7"
 	"code.gitea.io/gitea/models/migrations/v1_8"
 	"code.gitea.io/gitea/models/migrations/v1_9"
+	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
 
@@ -527,6 +529,13 @@ Please try upgrading to a lower version first (suggested v1.6.4), then upgrade t
 		return nil
 	}
 
+	// Some migration tasks depend on the git command
+	if git.DefaultContext == nil {
+		if err = git.InitSimple(context.Background()); err != nil {
+			return err
+		}
+	}
+
 	// Migrate
 	for i, m := range migrations[v-minDBVersion:] {
 		log.Info("Migration[%d]: %s", v+int64(i), m.Description())