Git: Fix reading of gitfile

* QFile ctor does not open the file for read.
* Replace readLine with read(8) to avoid needless read.

This amends commit e9eeaf33b3.

Change-Id: I33c230497ad431091d7d6b271846f5af661b4e0d
Reviewed-by: André Hartmann <aha_1980@gmx.de>

src/plugins/git/gitversioncontrol.cpp

@@ -82,7 +82,10 @@ bool GitVersionControl::isVcsFileOrDirectory(const Utils::FileName &fileName) co
         return false;
     if (fileName.toFileInfo().isDir())
         return true;
-    return QFile(fileName.toString()).readLine().startsWith("gitdir: ");
+    QFile file(fileName.toString());
+    if (!file.open(QFile::ReadOnly))
+        return false;
+    return file.read(8) == "gitdir: ";
 }
 
 bool GitVersionControl::isConfigured() const