fetch: improve the error messages emitted for conflicting refspecs

If we find two refspecs that want to update the same local reference,
emit an error message that is more informative based on whether one of
the conflicting refspecs is an opportunistic update during a fetch
with explicit command-line refspecs.  And especially, do not die if an
opportunistic reference update conflicts with an express wish of the
user; rather, just emit a warning and skip the opportunistic reference
update.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

remote.c

@@ -747,9 +747,28 @@ int for_each_remote(each_remote_fn fn, void *priv)
 
 static void handle_duplicate(struct ref *ref1, struct ref *ref2)
 {
-	if (strcmp(ref1->name, ref2->name))
-		die(_("%s tracks both %s and %s"),
-		    ref2->peer_ref->name, ref1->name, ref2->name);
+	if (strcmp(ref1->name, ref2->name)) {
+		if (ref1->fetch_head_status != FETCH_HEAD_IGNORE &&
+		    ref2->fetch_head_status != FETCH_HEAD_IGNORE) {
+			die(_("Cannot fetch both %s and %s to %s"),
+			    ref1->name, ref2->name, ref2->peer_ref->name);
+		} else if (ref1->fetch_head_status != FETCH_HEAD_IGNORE &&
+			   ref2->fetch_head_status == FETCH_HEAD_IGNORE) {
+			warning(_("%s usually tracks %s, not %s"),
+				ref2->peer_ref->name, ref2->name, ref1->name);
+		} else if (ref1->fetch_head_status == FETCH_HEAD_IGNORE &&
+			   ref2->fetch_head_status == FETCH_HEAD_IGNORE) {
+			die(_("%s tracks both %s and %s"),
+			    ref2->peer_ref->name, ref1->name, ref2->name);
+		} else {
+			/*
+			 * This last possibility doesn't occur because
+			 * FETCH_HEAD_IGNORE entries always appear at
+			 * the end of the list.
+			 */
+			die(_("Internal error"));
+		}
+	}
 	free(ref2->peer_ref);
 	free(ref2);
 }

t/t5536-fetch-conflicts.sh

@@ -22,7 +22,7 @@ verify_stderr () {
 	cat >expected &&
 	# We're not interested in the error
 	# "fatal: The remote end hung up unexpectedly":
-	grep -v "hung up" <error >actual &&
+	grep -E '^(fatal|warning):' <error | grep -v 'hung up' >actual | sort &&
 	test_cmp expected actual
 }
 
@@ -49,7 +49,7 @@ test_expect_success 'fetch conflict: config vs. config' '
 		cd ccc &&
 		test_must_fail git fetch origin 2>error &&
 		verify_stderr <<-\EOF
-		fatal: refs/remotes/origin/branch1 tracks both refs/heads/branch1 and refs/heads/branch2
+		fatal: Cannot fetch both refs/heads/branch1 and refs/heads/branch2 to refs/remotes/origin/branch1
 		EOF
 	)
 '
@@ -78,18 +78,22 @@ test_expect_success 'fetch conflict: arg vs. arg' '
 			refs/heads/*:refs/remotes/origin/* \
 			refs/heads/branch2:refs/remotes/origin/branch1 2>error &&
 		verify_stderr <<-\EOF
-		fatal: refs/remotes/origin/branch1 tracks both refs/heads/branch1 and refs/heads/branch2
+		fatal: Cannot fetch both refs/heads/branch1 and refs/heads/branch2 to refs/remotes/origin/branch1
 		EOF
 	)
 '
 
-test_expect_failure 'fetch conflict: criss-cross args' '
+test_expect_success 'fetch conflict: criss-cross args' '
 	setup_repository xaa \
 		"+refs/heads/*:refs/remotes/origin/*" && (
 		cd xaa &&
 		git fetch origin \
 			refs/heads/branch1:refs/remotes/origin/branch2 \
-			refs/heads/branch2:refs/remotes/origin/branch1
+			refs/heads/branch2:refs/remotes/origin/branch1 2>error &&
+		verify_stderr <<-\EOF
+		warning: refs/remotes/origin/branch1 usually tracks refs/heads/branch1, not refs/heads/branch2
+		warning: refs/remotes/origin/branch2 usually tracks refs/heads/branch2, not refs/heads/branch1
+		EOF
 	)
 '