Fix login redirection links (#17451)

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: zeripath <art27@cantab.net>

templates/base/head_navbar.tmpl

@@ -192,7 +192,7 @@
 					{{svg "octicon-person"}} {{.i18n.Tr "register"}}
 				</a>
 			{{end}}
-			<a class="item{{if .PageIsSignIn}} active{{end}}" rel="nofollow" href="{{AppSubUrl}}/user/login?redirect_to={{.CurrentURL}}">
+			<a class="item{{if .PageIsSignIn}} active{{end}}" rel="nofollow" href="{{AppSubUrl}}/user/login{{if not .PageIsSignIn}}?redirect_to={{.CurrentURL}}{{end}}">
 				{{svg "octicon-sign-in"}} {{.i18n.Tr "sign_in"}}
 			</a>
 		</div><!-- end anonymous right menu -->