ci: documentation: git reset only checked-in *wrap files before diffing

Otherwise it will abort and complain about not-checked wrap
files that got promoted by meson, such as:
error: pathspec 'subprojects/gvdb.wrap' did not match any file(s) known to git

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2126>

.gitlab-ci.yml

@@ -900,12 +900,12 @@ valgrind ges:
 
   - ninja -C build/ update_girs
   # Ignore modifications to wrap files made by meson
-  - git checkout subprojects/*.wrap
+  - git checkout $(git ls-files 'subprojects/*.wrap')
   - ./ci/scripts/check-diff.py "gir files"
 
   - ./gst-env.py ninja -C build/ plugins_doc_caches
   # Ignore modifications to wrap files made by meson
-  - git checkout subprojects/*.wrap
+  - git checkout $(git ls-files 'subprojects/*.wrap')
   - ./ci/scripts/check-diff.py
 
   - export GI_TYPELIB_PATH=$PWD/girs