Use git.HOME_PATH for Git HOME directory (#20114) (#20293)

Before, in #19732, the old home directory is not correct.
This PR introduces a new config option for git home: git.HOME_PATH,
which is default to %(APP_DATA_PATH)/home

And pass env GNUPGHOME to git command, force Gitea to use a stable GNUPGHOME directory

custom/conf/app.example.ini

@@ -617,7 +617,10 @@ ROUTER = console
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;
 ;; The path of git executable. If empty, Gitea searches through the PATH environment.
-PATH =
+;PATH =
+;;
+;; The HOME directory for Git
+;HOME_PATH = %(APP_DATA_PATH)/home
 ;;
 ;; Disables highlight of added and removed changes
 ;DISABLE_DIFF_HIGHLIGHT = false