Add option(s) to allow/disallow "dirty" uploads.

"Dirty" (we really need a better term - this is submodule terminology that is probably less than ideal) here means tar uploads from repositories that have uncommitted changes - so the generated git hash is imprecise.

See comments on https://github.com/galaxyproject/planemo/issues/170#issuecomment-98769271.

Start by reading the issue and the linked comment on issue 170 to understand the proposed behavior and terminology. Identify the upload entry point and existing option conventions, then define completion as allowing users to permit or reject tar uploads from repositories with uncommitted changes while preserving accurate status reporting.

Written by the indexing model from the issue text.