This is a bug report for the Mac OS X packaging at http://git-scm.com/download/mac . It is not so much a software bug as a nonoptimal configuration setting.
Requested Change Remove the line "excludesfile = ~/.gitignore" under [core] from /usr/local/git/etc/gitconfig
Reasons:
#1 The documentation at http://git-scm.com/docs/gitignore says the default global excludes file is at $HOME/.config/git/ignore. The file $HOME/.config/git/ignore is installed, and does indeed have good default content, but because of the setting in /usr/local/git/etc/gitconfig, the file is blocked from being reached.
#2 Seems to me the global excludesFile should never be called .gitignore -- better to be called .gitignore_global -- since the .gitignore name would prevent a local .gitignore from being included there. I personally don't have my $HOME directory under git control, but if I did, I might want a custom .gitignore. And if I did want that, I would be required to set my global file explicitly in my own ~/.gitconfig: git config --global core.excludesFile "~/.gitignore_global"