A couple issues with check-connectivity on fetch

Hi,

In the context of a remote helper using the `import` capability, or one
using the `fetch` capability but the without `check-connectivity`
capability, a large amount of time may be spent in check_connected() on
fetch, on large repos with a large number of fetched refs that also
happen not to actually add commits.

[A little bit of background: in git-cinnabar, which allows to talk to
Mercurial repositories, tags are fetched from a different remote, so
when one wants to fetch tags, the transport system never calls `import`
because the `list` ended up giving a large list of refs with resolved
sha1s, all of which are already locally available]

The first thing I noticed is that check_connected() is called twice
during the fetch: once from check_exist_and_connected(), and once from
store_updated_refs(). When each call is slow, it makes things doubly so.
I'm not sure why check_connected() is called twice like this.  It seems
it could be an oversight, but I could be missing something.

The second thing is that git-cinnabar uses the `import` capability,
and in that case, the transport layer never tried to use the
`check-connectivity` capability. I'm not sure whether the capability
should be exposed there, or if the checks should be skipped altogether
assuming `import` helpers use `fast-import` (although, technically,
git-cinnabar doesn't use the one provided by git).

But in any case, the `check-connectivity` capability doesn't actually
matter because no `fetch` or `import` even happens: fetch only does a
`list` and uses that result. In this case, it would seem
`check_connected` should be skipped entirely.

Here is a testcase that takes >20s to fetch on my machine (see
instructions at the beginning of the file)
http://glandium.org/files/git-check-connectivity-testcase
(the equivalent takes > 1 minute with git-cinnabar because of the
extra refs for git-cinnabar's metadata ; note the testcase is using
the `fetch` capability)

A workaround is to create temporary refs for those tags. That makes
check_connected()'s git-rev-list work quickly, and the fetch takes < 1s.

I'm ready to send patches to improve the situation, but I wanted some
feedback first about the right things that would need to change.

Thoughts?

Mike