Change detection using git "three dot" diff (#35)

* Rework change detection via `git diff`

Previous implementation performed simple diff between two versions. New implementation fetches on demand more commits to have the merge base between two branches. Now it will detect only changes introduced by branch that was pushed, instead of mixing with changes introduced meanwhile on the base branch.

tsconfig.json

@@ -1,6 +1,6 @@
 {
   "compilerOptions": {
-    "target": "es6",                          /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', 'ES2018', 'ES2019' or 'ESNEXT'. */
+    "target": "es2019",                          /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', 'ES2018', 'ES2019' or 'ESNEXT'. */
     "module": "commonjs",                     /* Specify module code generation: 'none', 'commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
     "outDir": "./lib",                        /* Redirect output structure to the directory. */
     "rootDir": "./src",                       /* Specify the root directory of input files. Use to control the output directory structure with --outDir. */