Add SSH user parameter (#1685)

* Add a configurable SSH user

* Update docs with param

* Indentation of readme

* formatting woes

* Update src/url-helper.ts

Co-authored-by: Josh Gross <joshmgross@github.com>

* Update action.yml

Co-authored-by: Josh Gross <joshmgross@github.com>

* Update genfiles

---------

Co-authored-by: Josh Gross <joshmgross@github.com>

README.md

@@ -62,6 +62,11 @@ Please refer to the [release page](https://github.com/actions/checkout/releases/
     # Default: true
     ssh-strict: ''
 
+    # The user to use when connecting to the remote SSH host. By default 'git' is
+    # used.
+    # Default: git
+    ssh-user: ''
+
     # Whether to configure the token or SSH key with the local git config
     # Default: true
     persist-credentials: ''