Init git module before database migration (#21764) (#21766)

Backport #21764

Some database migrations depend on the git module.

models/migrations/migrations.go

@@ -14,6 +14,7 @@ import (
 	"regexp"
 	"strings"
 
+	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
 
@@ -496,6 +497,13 @@ 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())