Git: Show upstream status for tracked branches

The Branch View now shows how many commmits
a branch is ahead and behind its tracking
branch.

Example:

  master ↑1 ↓15 [origin/master]

Change-Id: I9564efdfe82154cd98b9856313170aacad036f0c
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: André Hartmann <aha_1980@gmx.de>

src/plugins/git/gitclient.cpp

@@ -3037,6 +3037,15 @@ bool GitClient::synchronousSetTrackingBranch(const QString &workingDirectory,
             == SynchronousProcessResponse::Finished;
 }
 
+VcsBase::VcsCommand *GitClient::asyncUpstreamStatus(const QString &workingDirectory,
+                                                    const QString &branch,
+                                                    const QString &upstream)
+{
+    const QStringList args {"rev-list", noColorOption, "--left-right", "--count",
+                branch + "..." + upstream};
+    return vcsExec(workingDirectory, args, nullptr, false, silentFlags);
+}
+
 void GitClient::handleMergeConflicts(const QString &workingDir, const QString &commit,
                                      const QStringList &files, const QString &abortCommand)
 {