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 12039e00
[...]
 packet:        fetch> ref-prefix 12039e00
 packet:        fetch> ref-prefix refs/12039e00
 packet:        fetch> ref-prefix refs/tags/12039e00
 packet:        fetch> ref-prefix refs/heads/12039e00
 packet:        fetch> ref-prefix refs/remotes/12039e00
 packet:        fetch> ref-prefix refs/remotes/12039e00/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>

refspec.c

@@ -202,6 +202,8 @@ void refspec_ref_prefixes(const struct refspec *rs,
 		const struct refspec_item *item = &rs->items[i];
 		const char *prefix = NULL;
+		if (item->exact_sha1)
+			continue;
 		if (rs->fetch == REFSPEC_FETCH)
 			prefix = item->src;
 		else if (item->dst)

refspec.h

@@ -42,6 +42,10 @@ void refspec_clear(struct refspec *rs);
 int valid_fetch_refspec(const char *refspec);
 struct argv_array;
+/*
+ * Determine what <prefix> values to pass to the peer in ref-prefix lines
+ * (see Documentation/technical/protocol-v2.txt).
+ */
 void refspec_ref_prefixes(const struct refspec *rs,
 			  struct argv_array *ref_prefixes);

t/t5516-fetch-push.sh

@@ -1121,6 +1121,25 @@ test_expect_success 'fetch exact SHA1' '
 	)
 '
+test_expect_success 'fetch exact SHA1 in protocol v2' '
+	mk_test testrepo heads/master hidden/one &&
+	git push testrepo master:refs/hidden/one &&
+	git -C testrepo config transfer.hiderefs refs/hidden &&
+	check_push_result testrepo $the_commit hidden/one &&
+
+	mk_child testrepo child &&
+	git -C child config protocol.version 2 &&
+
+	# make sure $the_commit does not exist here
+	git -C child repack -a -d &&
+	git -C child prune &&
+	test_must_fail git -C child cat-file -t $the_commit &&
+
+	# fetching the hidden object succeeds by default
+	# NEEDSWORK: should this match the v0 behavior instead?
+	git -C child fetch -v ../testrepo $the_commit:refs/heads/copy
+'
+
 for configallowtipsha1inwant in true false
 do
 	test_expect_success "shallow fetch reachable SHA1 (but not a ref), allowtipsha1inwant=$configallowtipsha1inwant" '