issue a less intrusive warning for using an old version of git

When auto-generating a version number, git_helpers will issue a "git rev-list --count" command which can fail if the user has an old version of git that does not support the --count option.  The code detects this and will try a fall-back mechanism.  One side effect of this is that when "git rev-list --count" fails, the whole git rev-list usage message will get printed to standard out as warning.  This will occur every time "python setup.py" gets executed, even with "--help".  

As the fallback mechanism seems to work fine and I haven't seen any other ill effects of an older git version (I have to admit now that I have system where git is at 1.7.0.4.), I might suggest softening the warning message with the patch included here.  Perhaps others are aware of other pit falls to be aware of?