How to execute bash command in commit_message

@dereksmart
I want my commit message to contain a bash script to get a value from a file, is it doable?

Something like this
```yaml
- name: Push to staging branch
uses: Automattic/action-commit-to-branch@master
with:
branch: 'staging'
commit_message: ${ echo "Increase version to $(sed -rn 's/^version=([^\n]+)$/\1/p' file.properties)"}
```

Start by inspecting how the action reads the commit_message input and constructs the commit, then reproduce the example with file.properties in a test workflow. Done means a bash command can provide the value in the generated commit message, with the expected wording verified in the workflow output.

Written by the indexing model from the issue text.