Exclude .git/ from shellcheck (#7048)

If a branch name contains '.sh', current shellcheck checks the branch
file under .git/ and outputs error because the format is not shell
script one.
This makes shellcheck exclude files under .git/ to avoid this issue.

.gitlab-ci/shellcheck.yml

@@ -12,5 +12,5 @@ shellcheck:
     - shellcheck --version
   script:
     # Run shellcheck for all *.sh except contrib/
-    - find . -name '*.sh' -not -path './contrib/*' | xargs shellcheck --severity error
+    - find . -name '*.sh' -not -path './contrib/*' -not -path './.git/*' | xargs shellcheck --severity error
   except: ['triggers', 'master']