Use 'remote.origin.url' fallback for git < 1.7.5

Summary:
'git ls-remote --get-url' is more correct, but younger and less
supported. This commit tempers previous optimism about its availability,
improving support for users of older git packages.

Test Plan:

• Set git config url.xttps.insteadOf https rewrite rule.
• Ran arc which with git 1.7.5 in $PATH, saw rewritten configured remote.
• Ran arc which with git 1.7.4 in $PATH, saw configured remote.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D13998