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.

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