sync: tell apart same-path submanifest projects

Interleaved sync remembers which projects are done and which ones are
still pending by Project.relpath, and lists failing projects by it as
well. That path is relative to the project's own (sub)manifest though,
so once several manifests are synced together two projects can share
it, and the sets no longer tell them apart.

The damage is done when a project turns up in a later pass, e.g. a
submodule that is only derived once its parent has been synced: its
path is already recorded as finished, so it is left out of every pass
that follows and never synced, while sync still reports success. With
an outer manifest holding

  <project name="a" path="a" sync-s="true"/>

where a carries a submodule at b, and a submanifest at sub/ holding

  <project name="sub-ab" path="a/b"/>

a fresh `repo sync` checks out sub/a/b but never a/b. Failing projects
are reported under the same ambiguous path, and the stall detection
merges them too.

Use RelPath(local=opt.this_manifest_only) instead, which is unique
within the set of projects being synced and is what GetProjects() and
the other subcommands already use.

Spotted during the review of I30395b8a16a9154f60972e1f408a066af64ba77c.

Change-Id: I5fa694b968774667be6060d9722cb57acbd2b579
Signed-off-by: kimhappy <hwanhee.kim@laplacian.cc>
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/622641
Reviewed-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Brian Gan <brgan@google.com>