disambiguated target of Git operations

this avoids warnings like the following:

    fatal: ambiguous argument <...>: unknown revision or path not in the
    working tree.
    Use '--' to separate paths from revisions, like this:
    'git <command> [<revision>...] -- [<file>...]'

note that this requires an extra check to avoid deleted tiddlers'
revisions resurfacing (since revisions are never erased from Git's
history)