git config --global core.excludesfile
wasn't working. Turns out there were spaces at the end of the pattern line that were preventing it from working. According to the docs, spaces at the end of lines aren't supposed to matter:
Trailing spaces are ignored unless they are quoted with backlash ("\")
However, it did in this case. Perhaps this is just an issue with Git on Windows.
TIP: Use the following to test out your ignore patterns in dry run mode via -n:
git add -n *