Summary:
We can use `.gitattributes` instead but there's no way how to set repository config for all users in Git, right?
So provide a script writting to `.git/info/attributes` instead so that we don't have to .gitignore `.gitattributes`.
Test Plan:
$ scripts/celerity/install_merge.sh
$ git pull # with merge conflict in Celerity map
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin
Differential Revision: https://secure.phabricator.com/D3228