git: fetch submodules during Update Project (IDEA-200864)

`git submodule update` doesn't fetch submodules.
`git fetch` in the parent repository fetches submodules "on-demand", i.e. only if the parent repository references to unknown commit in the submodule.

It is safe to pass all repositories to the fetch support: the ones without a valid remote will be ignored.

plugins/git4idea/src/git4idea/update/GitUpdateProcess.java

@@ -132,7 +132,7 @@ public class GitUpdateProcess {
       return GitUpdateResult.NOT_READY;
     }
 
-    if (!fetchAndNotify(trackedBranches.keySet())) {
+    if (!fetchAndNotify(myRepositories)) {
       return GitUpdateResult.NOT_READY;
     }