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.com/D3228