Automatically recover from git hangs

It happens on occasion that the `git' command may hang indefinitely when interacting with a remote server.

For example, see http://pulse.test.qt.nokia.com:8080/browse/projects/QtBase%20master%20Integration/builds/596/logs/stage/linux-g%2B%2B-32%20Ubuntu%2010.04%20x86/raw/true/ , where git hung on cloning v8 for >12 hours.

We should find a way to detect (and ideally recover from) this. Note that we do already have logic to recover from various network problems while running git, implemented as part of QTQAINFRA-223. But nothing causes that logic to kick in if git hangs rather than exiting with an error.