Cover the core `repo init` & `repo sync` options (current branch only,
clone bundles, shallow clones, groups, local manifests & manifest
names), multi-step workflows (two-phase syncs, mirrors, incremental
syncs, local work, switching manifest branches, selfupdate, ...), and
manifests that broke users in the past. Some tests also run under Git 3
defaults.
Add `RepoClient` helpers to list remote-tracking branches & checkouts,
and document the test layout in the README.
Bug: 566270944
Change-Id: Iac333216e0f27e489e64f4eb853189f860a91f20
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/648544
Tested-by: Brian Gan <brgan@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Brian Gan <brgan@google.com>
