VcsCommand: Limit the usage of setCookie()

Don't use setCookie() for setting the push failure status.
Introduce a PushHandler helper that keeps the data collected
when stdErrText() signal is emitted and reuse it when
finished() is being called. Move m_pushFallbackCommand
field into the helper, as its value is meaningful only
in context of the invoked command.

Hide PushFailure enum inside PushHandler.

Change-Id: Id21457734f32e06bb130c3c07c24cb2b7af6e466
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/git/gitclient.h

@@ -80,12 +80,6 @@ enum StashFlag {
     NoPrompt       = 0x02
 };
 
-enum PushFailure {
-    Unknown,
-    NonFastForward,
-    NoRemoteBranch
-};
-
 class SubmoduleData
 {
 public:
@@ -425,7 +419,6 @@ private:
 
     QString m_gitQtcEditor;
     QMap<Utils::FilePath, StashInfo> m_stashInfo;
-    QString m_pushFallbackCommand;
     QString m_diffCommit;
     Utils::FilePaths m_updatedSubmodules;
     bool m_disableEditor = false;