Pass workflow dispatch inputs as strings

Even if it is tempting to pass `false` as a Boolean value, this is
incorrect. It will result in something like this:

{
  statusCode: 422,
  statusMessage: 'Unprocessable Entity',
  requestMethod: 'POST',
  requestPath: '/repos/git-for-windows/git-for-windows-automation/actions/workflows/tag-git.yml/dispatches',
  json: {
    message: "Invalid request.\n\nFor 'additionalProperties', false is not a string.",
    documentation_url: 'https://docs.github.com/rest/reference/actions#create-a-workflow-dispatch-event'
  }
}

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

GitForWindowsHelper/slash-commands.js

@@ -311,7 +311,7 @@ module.exports = async (context, req) => {
                     rev,
                     owner,
                     repo,
-                    snapshot: false
+                    snapshot: 'false'
                 }
             )