manifest_xml: improve topdir accuracy.
manifest_xml: improve topdir accuracy.

Do not include a trailing path separator when submanifest_path is empty.

Bug: https://crbug.com/gerrit/16104
Change-Id: Ia65e355de25bdb1067fe50ab1d47db6e798d5a71
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/343674
Tested-by: LaMont Jones <01939f9990a33aa2509cfaed21d2f9ba201a2b37@google.com>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@google.com>

GerritCodeReview/git-repo,GerritCodeReview/git-repo,lewixliu/git-repo,lewixliu/git-repo