R1:ff8b48979eb7

Simplify Repository remote and local command construction

Summary:
This cleans up some garbage:

- We were specifying environmental variables with `X=y git ...`, but now have `setEnv()` on both `ExecFuture` and `PhutilExecPassthru`. Use `setEnv()`.
- We were specifying the working directory with `(cd %s && git ...)`, but now have `setCWD()` on both `ExecFuture` and `PhutilExecPassthru`. Use `setCWD()`.
- We were specifying the Git credentials with `ssh-agent -c (ssh-add ... && git ...)`. We can do…