Description of problem:
Update with --update-md-path keeps baseurl - and it is not a good idea

Version-Release number of selected component (if applicable):
createrepo-0.9.9-12.fc18

How reproducible:
Always

Steps to Reproduce:

1) Prepare first repo which uses baseurl:
$ mkdir a_repo && cd a_repo && wget http://mirrors.kernel.org/fedora/releases/17/Fedora/i386/os/Packages/s/symlinks-1.4-5.fc17.i686.rpm && createrepo . --baseurl http://foo.bar/ && cd ..

2) I want to create another new repo:
$ mkdir b_repo
$ cd b_repo
$ wget http://mirrors.kernel.org/fedora/releases/17/Fedora/i386/os/Packages/s/symlinks-1.4-5.fc17.i686.rpm

3) I want to speed up creation of new repo with usage an existing metadata from some other repository. So I use:
$ createrepo --update-md-path /tmp/createrepo_bug/a_repo/ --update .
  
Actual results:
b_repo has in metadata this:
<location xml:base="http://foo.bar/" href="symlinks-1.4-5.fc17.i686.rpm"/>

Expected results:
b_repo has in metadata this:
<location href="symlinks-1.4-5.fc17.i686.rpm"/>
because I only want to speed up repo creation, I have all packages locally, I don't want to baseurl in my output metadata.
I suppose that createrepo during update should use and keep only useful data from remote/old repo. As useful I consider all data except the baseurl :)