R1:5c21b5345d28

execx ==> execxLocalCommand for git libraries in diffusion

Summary:
this was fairly mechanical at the end of the day

note that future/exec got removed by the code generation robots
post this change

Test Plan:
clicked around diffusion a bunch looking for errors.

For a given repo (say http://phabricator.dev/diffusion/BOBALIE/)
- http://phabricator.dev/diffusion/BOBALIE/history/
- http://phabricator.dev/diffusion/BOBALIE/history/origin:master/.arcconfig
- http://phabricator.