remote-hg: do not fail on invalid bookmarks

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>