git_mirror.pl should try hard to recover from transient errors

On certain repos on certain servers, a git clone will often fail. git://gitorious.org/webkit/qtwebkit.git is a good example:

$ git clone git://gitorious.org/webkit/qtwebkit.git
fatal: The remote end hung up unexpectedly
fatal: early EOF
fatal: index-pack failed

Ideally we are able to have all such problems fixed. But, to be pragmatic,
our git wrapper script git_mirror.pl should automatically retry on errors of this form.