clean up some git basics

data/git/subtree.md

@@ -0,0 +1,34 @@
+---
+title: "git"
+tags: [ "Documentation", "data", "git", "subtree" ]
+---
+
+## Pulling a Subtree from an existing git
+
+The project has subdirectories `sub-1`, `sub-2`, `sub-3`.
+The first should be its own repository, but should also retain its own history.
+
+First, we extract its history as an independent item, and make that into a seprate branch.
+
+```bash
+git subtree split --prefix=sub-1 -b sub
+```
+
+If you want something a few directories deep, you can use `--prefix=sub-1/dir-2/dir-3
+
+Then go and create a new git somewhere else:
+
+```bash
+cd ..;mkdir sub-1;cd sub-1;git init --bare
+```
+
+Then go back to your initial git repo, and do the following:
+
+git push ../subtest sub:master
+
+Finally, you can clone this repo from your original.
+
+```bash
+git clone ../subtest
+```
+