Merge branch 'fc/fetch-with-import-fix'

Merge branch 'fc/fetch-with-import-fix'

Code restructuring during 2.20 period broke fetching tags via
"import" based transports.

* fc/fetch-with-import-fix:
  fetch: fix regression with transport helpers
  fetch: make the code more understandable
  fetch: trivial cleanup
  t5801 (remote-helpers): add test to fetch tags
  t5801 (remote-helpers): cleanup refspec stuff