In "arc land", when "remote/onto" does not exist locally, try to fetch it before giving up

Summary:
Fixes T10650. It's valid to arc land --remote origin --onto master without first fetching that ref. If we can't find a local ref for a specified remote branch, try to fetch it before giving up.

(In the long run, this should be valid even if the remote branch does not exist at all and the user intends to create it -- see T12876 -- but this is a step toward that.)

Test Plan:

• Ran rm .git/refs/remotes/origin/master, then landed into "master".
• Before: "arc land" bailed out immediately.
• After: "arc land" fetches the missing ref.

$ arc land
 TARGET  Landing onto "master", the default target under git.
 REMOTE  Using remote "origin", the default remote under Git.
 TARGET  No local ref exists for branch "master" in remote "origin", attempting fetch...
 FETCHED  Fetched branch "master" from remote "origin".
...

Maniphest Tasks: T10650

Differential Revision: https://secure.phabricator.com/D20870