I noticed that git recently stopped obeying the ignore patterns from the global core.excludefiles specification file. Has anyone else noticed that too? If it is not just me then I am wondering if it's related to the recent upgrade to git 2.7.

Best to check by using pacman -U from your pacman cache and downgrade to the previous (2.6) version.

git-2.6.5 doesn't have this issue, so must be a regression in 2.7. Hopefully a fix will be released soon.