Ignoring local changes to a file in git

See http://stackoverflow.com/questions/1753070/git-ignore-files-only-locally

Basically, edit .git/config/exclude.

e.g.
# git ls-files --others --exclude-from=.git/info/exclude
# Lines that start with '#' are comments.
# For a project mostly in C, the following would be a good set of
# exclude patterns (uncomment them if you want to use them):
# *.[oa]
# *~
sites/default/settings.php


Then run:
git update-index --assume-unchanged sites/default/settings.php
for the changes to take effect.