Merge branch 'jc/transport-do-not-use-connect-twice-in-fetch' into maint

The auto-tag-following code in "git fetch" tries to reuse the same
transport twice when the serving end does not cooperate and does
not give tags that point to commits that are asked for as part of
the primary transfer.  Unfortunately, Git-aware transport helper
interface is not designed to be used more than once, hence this
does not work over smart-http transfer.

* jc/transport-do-not-use-connect-twice-in-fetch:
  builtin/fetch.c: Fix a sparse warning
  fetch: work around "transport-take-over" hack
  fetch: refactor code that fetches leftover tags
  fetch: refactor code that prepares a transport
  fetch: rename file-scope global "transport" to "gtransport"
  t5802: add test for connect helper