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 "fetch" failed (no credentials passed).
• Applied this patch.
• Re-ran "phd debug fetch <phid>", verified it passed credentials and

succeeded.

• Did all these steps for a Mercurial repo.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, btrahan

Maniphest Tasks: T686

Differential Revision: 1236