#17 Git repo linking support

There are various tools out there for linking Git repos together. * The most obvious and integrated one are submodules: #13 * `git-repo` is a Google tool (used e.g. on AOSP) * https://gerrit.googlesource.com/git-repo * https://source.android.com/docs/setup/download * `depot_tools`'s `fetch`/`gclient`, another Google tool (used e.g. on Chromium) * https://chromium.googlesource.com/chromium/tools/depot_tools * `gil` (git links): https://github.com/chronoxor/gil * Supports recursive structures. https://github.com/andry81-tests/gil-test1-A would result in an infinite loop in a naive implementation. * `repobuild` (seems to be based on submodules): https://github.com/chrisvana/repobuild * Probably many more; I've seen references to Buck (Facebook) and Pants (Twitter) but couldn't quickly determine whether they're relevant or not. `gil` illustrates a general problem with such approaches: they may result in loops, resolving of which requires maintaining global state across repos. This doesn't fit well into codearchiver's concept of isolated module runs that know nothing about the parent state. Also, anything like this would have to hook directly into the Git module, or possibly even cross-VCS.