Git: Do not require the path to be set in the config

Fix regression where creator was no longer able to run git if the
PATH was not explicitly set in the configuration.

Task-number: QTCREATORBUG-2478
Reviewed-by: Robert Loehning

src/plugins/git/gitsettings.h

@@ -47,6 +47,7 @@ struct GitSettings
     void fromSettings(QSettings *);
     void toSettings(QSettings *) const;
 
+    /** Return the full path to the git executable */
     QString gitBinaryPath(bool *ok = 0, QString *errorMessage = 0) const;
 
     bool equals(const GitSettings &s) const;