radicle-git init

Port over all git relevant parts of the radicle-link repository[0].
The packages that are kept are:
* git-ext
* git-ref-format
* git-trailers
* link-git
* macros
* std-ext
* test -- only the portions that test the above

The development and testing scripts are also added for local and CI
testing.

[0]: https://github.com/radicle-dev/radicle-link/
Signed-off-by: Fintan Halpenny <fintan.halpenny@gmail.com>

scripts/ci/macos-gnu

@@ -0,0 +1,5 @@
+if brew ls --versions gnu-tar > /dev/null; then
+  echo "gnu-tar is already installed"
+else
+  brew install gnu-tar
+fi