Never merge using rebase during a git pull

My .gitconfig setting branch.autosetuprebase=always caused "git pull" to
attempt to rebase merge, which we never want, because we don't want to
re-write history. Passing --no-rebase on the command line avoids rebase.

Auditors: csilvers

Test Plan:
I ran also removed the "git push" line following the "git pull", then
ran ./update_phabricator.sh and saw that it did not try (and fail) to
rebase the changes in "origin" on top of those from "upstream".