replace "(cd path && git cmd)" with "git -C path cmd" This matches the existing use of "got -r path cmd" and "git_commit path args".

add an initial implementation of gotadmin dump gotadmin dump is used to export (part of) the history of the repository; at the moment it only generates git bundles (which are pack files with a header) but support to generate a fast-import stream is planned. ok/tweaks stsp and jamsek