Allow for removing path prefixes on the client

[thmo]

To be able to mirror server's foo/bar.git repo into /target/dir/bar.git on the client, an option to strip the initial foo/ part would be useful.

Note though, that this could lead to name clashes on the client, so the client would have to check for duplicates in the set of repositories to be written before actually doing the pull, and bail out.

[mricon]

This would be tricky in many ways, due to the way references and symlinks work. I can see how this feature would be useful, but it's a non-trivial change.