* git/fetch.scm (make-fetch-options): Add #:transfer-progress and honor
it.  Add docstring.
(set-fetch-options-transfer-progress!): New procedure.
* git/structs.scm (%indexer-progress): New variable.
(<indexer-progress>): New record type.
(bytestructure->indexer-progress, procedure->indexer-progress-callback)
(set-remote-callbacks-transfer-progress!): New procedures.
* tests/clone.scm ("clone + transfer-progress"): New test.