fix(ng-dev): merge tool accidentally performing unauthenticated Github requests

[devversion]

The merge tool accidentally performs unauthenticated Github API
requests. This started happening when we refactored some things
in the merge tool.

We should not rely on default values for parameters as explicit
parameters encourage thinking about passing the potentially needed
/already-existing values (like for the authenctiated git client).

This commit changes this in all places of the merge tool and
release tool in order to ensure we always use the correct
instance of the Git client (as initially configured).

Note: We could theoretically have a method that returns a git
client singleton, prioritizing the authenticated one. Though I
personally still believe that being explicit is better and encouraging
thinking of function consumers.

RequestError [HttpError]: API rate limit exceeded for <ip>. (But here's the good news: Authenticated requests get a higher rate limit. Check out the documentation for more details.)
  at /usr/local/google/home/wagnermaciel/Documents/angular/components/node_modules/@octokit/request/dist-src/fetch-wrapper.js:68:27
  at processTicksAndRejections (internal/process/task_queues.js:95:5)
  at async getVersionOfBranch (/usr/local/google/home/wagnermaciel/Documents/angular/components/ng-dev/release/versioning/version-branches.ts:51:18)
  at async fetchActiveReleaseTrains (/usr/local/google/home/wagnermaciel/Documents/angular/components/ng-dev/release/versioning/active-release-trains.ts:34:23)
  at async getTargetLabelsForActiveReleaseTrains (/usr/local/google/home/wagnermaciel/Documents/angular/components/ng-dev/pr/merge/defaults/labels.ts:54:44)
  at async getTargetBranchesForPullRequest (/usr/local/google/home/wagnermaciel/Documents/angular/components/ng-dev/pr/merge/target-label.ts:112:26)
  at async loadAndValidatePullRequest (/usr/local/google/home/wagnermaciel/Documents/angular/components/ng-dev/pr/merge/pull-request.ts:73:26)
  at async PullRequestMergeTask.merge 

[josephperrott]

LGTM

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