[ADD] doc unswallow a git repo

doc/GIT_REPO.md

@@ -125,3 +125,11 @@ Maybe, some version diverge from your manifest. Simply clean all and relaunch yo
 ```bash
 ./script/clean_repo_manifest.sh
 ```
+
+### Unshallow git
+
+By example, the repo Odoo use a depth clone. If you need all the clone repo, use this command on right directory:
+
+```bash
+git fetch REMOTE --unshallow
+```