Make updater faster

[n-dusan]

Running updater on a top-level repository with dependencies.json is very slow. Profile the updater and find hot code that takes up the bulk of the update time.

• [ ] Profiling framework (benchmark) on taf updater on no-op operation
  • [ ] benchmark for no-op update
  • [ ] benchmark for an update
• [x] When validating repositories, instead of cloning from remote repository to %localappdata%\Temp dir, clone from local disk into Temp dir. After the repository is validated, run git pull
• [ ] Validate each branch, then merge all 50 commits at once, instead of commit after commit for target repos
• [x] LOW PRIORITY: It should be possible to parallelize work to separate processes, where each dependency repository is a separate worker/thread/process.

[renatav]

We should create a script or a test which just fetches from remote and compare its performance to the updater's. If there are no new changes, the updater should not be much slower than this script.

[renatav]

re merging commits: we merge one branch at a time. I am not sure how to speed this up. Let's check how slow this.