Update "git rev-parse" invocation to work in Git 2.25.0

Summary:
Fixes T13479. The behavior of "git rev-parse --show-toplevel" has changed in Git 2.25.0, and it now fails in bare repositories.

Instead, use "git rev-parse --git-dir" to sanity-check the working copy. This appears to have more stable behavior across Git versions, although it's a little more complicated for our purposes.

Test Plan:

• Ran bin/repository update ... on an observed, bare repository.
• ...on an observed, non-bare ("legacy") repository.
• ...on a hosted, bare repository.

Maniphest Tasks: T13479

Differential Revision: https://secure.phabricator.com/D20945