VCS: Simplify promptSubmit a bit more

+ fix regression for Git, which allowed to commit invalid commits.

Amends commit 9b61e484be.

Change-Id: I7571863bb982bfae1aa040e08f8c68010b14b147
Reviewed-by: André Hartmann <aha_1980@gmx.de>

src/plugins/git/gitplugin.cpp

@@ -252,6 +252,7 @@ public:
     void updateActions(VcsBasePluginPrivate::ActionState) override;
     bool submitEditorAboutToClose() override;
     void discardCommit() override { cleanCommitMessageFile(); }
+    bool canCommitOnFailure() const override { return false; }
 
     void diffCurrentFile();
     void diffCurrentProject();