techdocs: Replace nodegit with isomorphic-git

TechDocs needs 4 functions from the git library
1. To clone 2. To get the commit timestamp for HEAD 3. To fetch 4. To merge two branches

Created necessary commands in the new SCM client and reordered them alphabetically for convenience

packages/techdocs-common/package.json

@@ -50,7 +50,6 @@
     "js-yaml": "^3.14.0",
     "mime-types": "^2.1.27",
     "mock-fs": "^4.13.0",
-    "nodegit": "^0.27.0",
     "recursive-readdir": "^2.2.2",
     "winston": "^3.2.1"
   },
@@ -61,7 +60,6 @@
     "@types/js-yaml": "^3.12.5",
     "@types/mime-types": "^2.1.0",
     "@types/mock-fs": "^4.13.0",
-    "@types/nodegit": "^0.26.12",
     "@types/recursive-readdir": "^2.2.0"
   },
   "jest": {