Add isomorphic git to project, and allow different bases (#8)

* Add isomorphic-git to the project

This required the typescript module resolution
mode.

* Allow for different types of bases

Allow for tags, either the same or another branch,
or a commit hash to be used as a base.

* Add tests for using commits / tags as base

* Fix bug using tag as base

Author: s0

Diff for: package.json

@@ -68,5 +68,8 @@
   "packageManager": "[email protected]",
   "publishConfig": {
     "access": "public"
+  },
+  "dependencies": {
+    "isomorphic-git": "^1.27.1"
   }
 }