error: git checkout-index: unable to write file

Hi gitsters,

Just tried cloning an enormous repo to /tmp (which uses tmpfs) and got

 error: git checkout-index: unable to write file webkit/tools/test_shell/resources/textAreaResizeCorner.png
 error: git checkout-index: unable to write file webkit/tools/test_shell/run_all_tests.cc
 error: git checkout-index: unable to write file webkit/tools/test_shell/simple_appcache_system.cc
 error: git checkout-index: unable to write file webkit/tools/test_shell/simple_appcache_system.h

[etc].

Maybe we can do better?  It seems non-ideal in a few ways:

 1) it is not checkout-index but read-tree -m that prints the message
 2) errno is forgotten
 3) I would prefer if "git clone" errored out quickly for ENOSPC instead
    of continuing to fill in the cracks with what small files it can
    find.  Though I suppose the current behavior is better for errors
    like EPERM and ENAMETOOLONG, so I can't complain much.