R1:b5c9b9d059dc

Use remote credentials for 'git fetch' and 'hg pull' commands

Summary: These are "local" commands, but need remote credentials. If the daemon
runs as a user who does not have credentials, the initial clone will work but
subsequent updates will fail.

Test Plan:
- Nuked a local copy of a Git repo.
- Ran "phd debug fetch <phid>" as root (or any other user with no natural SSH
keys). Verified initial clone worked (since it passes credentials to the command
correctly).
- Killed daemon, re-ran, verified…