Cover `repo sync -n` & `-l` on existing checkouts (interleaved &
phased), projects that were never fetched, and two-phase syncs with a
superproject. `-l` must not touch the network, which the tests check by
taking the server offline. Also cover tags (`--no-tags` & `sync-tags`),
`--prune`, `--force-sync`, `--force-remove-dirty` & `-d`.
Teach the harness to simulate outages (`GitHost.offline`) and to collect
the requests made during a block (`GitHost.watch_requests`), and add a
few `RepoClient` helpers.
Bug: 566270944
Change-Id: I96b4729a7d4e5b58bc57fb46a060beda35549979
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/648545
Tested-by: Brian Gan <brgan@google.com>
Commit-Queue: Brian Gan <brgan@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
