Made gitBlob() static and execute in /tmp as this command can be executed

for every file not only the files contained in a git repository.

git/git_client.cpp

@@ -330,7 +330,7 @@ QString GitClient::gitBlob(QString fileName) {
     if (fi.exists()) {
         QString const gitCommand = QString("git hash-object %1").arg(fileName);
         Command c(gitCommand);
-        if (c.execute(m_workingDirectory)) {
+        if (c.execute("/tmp")) {
             return c.getCommandResult().trimmed();
         }
     }