ensure content files are available in config

If you use the cli with the `--content` option, then we first resolve
the config (empty), then add the `content` to the config. The issue is
that this means that the content will be empty when you resolve it
initially. This results in a warning in your terminal.

Now, we will make sure to merge 2 configs if you have the `--content`
data passed. We will also make sure to override the final
`config.content.files` to whatever you passed in to make sure that this
is the same behaviour as before.

Author: RobinMalfait

src/cli.js

@@ -437,7 +437,6 @@ async function build() {
 
   function resolveConfig() {
     let config = configPath ? require(configPath) : {}
-    let resolvedConfig = resolveConfigInternal(config)
 
     if (args['--purge']) {
       log.warn('purge-flag-deprecated', [
@@ -450,10 +449,13 @@ async function build() {
     }
 
     if (args['--content']) {
-      resolvedConfig.content.files = args['--content'].split(/(?<!{[^}]+),/)
+      let files = args['--content'].split(/(?<!{[^}]+),/)
+      let resolvedConfig = resolveConfigInternal(config, { content: { files } })
+      resolvedConfig.content.files = files
+      return resolvedConfig
     }
 
-    return resolvedConfig
+    return resolveConfigInternal(config)
   }
 
   function extractFileGlobs(config) {