Fix: "git rm" fails when publish branch is empty (#24)

When a publish branch exists and it has no files, `git rm -r '*'` fails with

```
fatal: pathspec '*' did not match any files
```

`--ignore-unmatch` option let git exit with a zero status in this case.

Reference: https://git-scm.com/docs/git-rm#Documentation/git-rm.txt---ignore-unmatch

entrypoint.sh

@@ -55,7 +55,7 @@ remote_branch="${PUBLISH_BRANCH}"
 local_dir="${HOME}/$(tr -cd 'a-f0-9' < /dev/urandom | head -c 32)"
 if git clone --depth=1 --single-branch --branch "${remote_branch}" "${remote_repo}" "${local_dir}"; then
     cd "${local_dir}"
-    git rm -r '*'
+    git rm -r --ignore-unmatch '*'
     find "${GITHUB_WORKSPACE}/${PUBLISH_DIR}" -maxdepth 1 | \
         tail -n +2 | \
         xargs -I % cp -rf % "${local_dir}/"