In "arc land" in Mercurial, show a tidier "ls-remote" command

Summary:
Ref T9948. Ref T13546. We must passthru "hg ls-remote" because it might prompt the user for credentials.

Since "ls-remote" is implemented as an exension and we can't rely on using stdout beacuse of passthru, the actual command we execute is:

$ hg --config extensions.arc-hg=<huge long path to extension> arc-ls-remote --output <huge long path to temporary file> -- remote

This is meaningless and distracting; show the intent of the command we're executing instead. Users can see the raw command in "--trace" if they're actually debugging behavior.

Test Plan: Ran "arc land" in a Mercurial repository, got a tidier command output.

Maniphest Tasks: T13546, T9948

Differential Revision: https://secure.phabricator.com/D21344