fetch: do not pass ref-prefixes for fetch by exact SHA1

fetch: do not pass ref-prefixes for fetch by exact SHA1

When v2.18.0-rc0~10^2~1 (refspec: consolidate ref-prefix generation
logic, 2018-05-16) factored out the ref-prefix generation code for
reuse, it left out the 'if (!item->exact_sha1)' test in the original
ref-prefix generation code. As a result, fetches by SHA-1 generate
ref-prefixes as though the SHA-1 being fetched were an abbreviated ref
name:

 $ GIT_TRACE_PACKET=1 bin-wrappers/git -c protocol.version=2 \
	fetch origin 12039e008f9a4e3394f3f94f8ea897785cb09448
[...]
 packet:        fetch> ref-prefix 12039e008f9a4e3394f3f94f8ea897785cb09448
 packet:        fetch> ref-prefix refs/12039e008f9a4e3394f3f94f8ea897785cb09448
 packet:        fetch> ref-prefix refs/tags/12039e008f9a4e3394f3f94f8ea897785cb09448
 packet:        fetch> ref-prefix refs/heads/12039e008f9a4e3394f3f94f8ea897785cb09448
 packet:        fetch> ref-prefix refs/remotes/12039e008f9a4e3394f3f94f8ea897785cb09448
 packet:        fetch> ref-prefix refs/remotes/12039e008f9a4e3394f3f94f8ea897785cb09448/HEAD
 packet:        fetch> 0000

If there is another ref name on the command line or the object being
fetched is already available locally, then that's mostly harmless.
But otherwise, we error out with

 fatal: no matching remote head

since the server did not send any refs we are interested in.  Filter
out the exact_sha1 refspecs to avoid this.

This patch adds a test to check this behavior that notices another
behavior difference between protocol v0 and v2 in the process.  Add a
NEEDSWORK comment to clear it up.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>