remote-hg: do not fail on invalid bookmarks

Mercurial can have bookmarks pointing to "nullid" (the empty root
revision), while Git can not have references to it. When cloning or
fetching from a Mercurial repository that has such a bookmark, the
import failed because git-remote-hg was not be able to create the
corresponding reference.

Warn the user about the invalid reference, and do not advertise these
bookmarks as head refs, but otherwise continue the import. In
particular, we still keep track of the fact that the remote repository
has a bookmark of the given name, in case the user wants to modify that
bookmark.

Also add some test cases for this issue.

Reported-by: Antoine Pelisse <apelisse@gmail.com>
Signed-off-by: Max Horn <max@quendi.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/remote-helpers/git-remote-hg

@@ -641,7 +641,10 @@ def do_list(parser):
             print "? refs/heads/branches/%s" % gitref(branch)
 
     for bmark in bmarks:
-        print "? refs/heads/%s" % gitref(bmark)
+        if  bmarks[bmark].hex() == '0000000000000000000000000000000000000000':
+            warn("Ignoring invalid bookmark '%s'", bmark)
+        else:
+            print "? refs/heads/%s" % gitref(bmark)
 
     for tag, node in repo.tagslist():
         if tag == 'tip':

contrib/remote-helpers/test-hg.sh

@@ -691,4 +691,77 @@ test_expect_success 'remote double failed push' '
 	)
 '
 
+test_expect_success 'clone remote with master null bookmark, then push to the bookmark' '
+	test_when_finished "rm -rf gitrepo* hgrepo*" &&
+
+	hg init hgrepo &&
+	(
+		cd hgrepo &&
+		echo a >a &&
+		hg add a &&
+		hg commit -m a &&
+		hg bookmark -r null master
+	) &&
+
+	git clone "hg::hgrepo" gitrepo &&
+	check gitrepo HEAD a &&
+	(
+		cd gitrepo &&
+		git checkout --quiet -b master &&
+		echo b >b &&
+		git add b &&
+		git commit -m b &&
+		git push origin master
+	)
+'
+
+test_expect_success 'clone remote with default null bookmark, then push to the bookmark' '
+	test_when_finished "rm -rf gitrepo* hgrepo*" &&
+
+	hg init hgrepo &&
+	(
+		cd hgrepo &&
+		echo a >a &&
+		hg add a &&
+		hg commit -m a &&
+		hg bookmark -r null -f default
+	) &&
+
+	git clone "hg::hgrepo" gitrepo &&
+	check gitrepo HEAD a &&
+	(
+		cd gitrepo &&
+		git checkout --quiet -b default &&
+		echo b >b &&
+		git add b &&
+		git commit -m b &&
+		git push origin default
+	)
+'
+
+test_expect_success 'clone remote with generic null bookmark, then push to the bookmark' '
+	test_when_finished "rm -rf gitrepo* hgrepo*" &&
+
+	hg init hgrepo &&
+	(
+		cd hgrepo &&
+		echo a >a &&
+		hg add a &&
+		hg commit -m a &&
+		hg bookmark -r null bmark
+	) &&
+
+	git clone "hg::hgrepo" gitrepo &&
+	check gitrepo HEAD a &&
+	(
+		cd gitrepo &&
+		git checkout --quiet -b bmark &&
+		git remote -v &&
+		echo b >b &&
+		git add b &&
+		git commit -m b &&
+		git push origin bmark
+	)
+'
+
 test_done