R1:80d9a276f321

Automatically resolve merge conflicts in Celerity map

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.