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.

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.

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

git-remote-hg

@@ -648,7 +648,10 @@ 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() == '0' * 40:
+            warn("Ignoring invalid bookmark '%s'", bmark)
+        else:
+            print "? refs/heads/%s" % gitref(bmark)
 
     for tag, node in repo.tagslist():
         if tag == 'tip':

test/main.t

@@ -773,4 +773,29 @@ test_expect_success 'remote double failed push' '
 	)
 '
 
+test_expect_success 'clone remote with null bookmark, then push' '
+	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 bookmark
+	) &&
+
+	(
+	git clone "hg::hgrepo" gitrepo &&
+	check gitrepo HEAD a &&
+	cd gitrepo &&
+	git checkout --quiet -b bookmark &&
+	git remote -v &&
+	echo b > b &&
+	git add b &&
+	git commit -m b &&
+	git push origin bookmark
+	)
+'
+
 test_done