Further correct and disambigutate ref selectors passed to Git on the CLI

Summary:
Ref T13589. In D21510, not every ref selector got touched, and this isn't a valid construction in Git:

$ git ls-tree ... -- ''

Thus:

• Disambiguate more (all?) ref selectors.
• Correct the construction of "git ls-tree" when there is no path.
• Clean some stuff up: make the construction of some flags and arguments more explicit, get rid of a needless "%C", prefer "%Ls" over acrobatics, etc.

Test Plan: Browsed/updated a local Git repository. (This change is somewhat difficult to test exhaustively, as evidenced by the "ls-tree" issue in D21510.)

Maniphest Tasks: T13589

Differential Revision: https://secure.phabricator.com/D21511