manifest: set revisionId as revision attribute it it is not being set in ToXml.

As we were testing superproject setting revisionId attribute to SHA and
reloading the manifest, we found out revisionId attribute is not being
saved. Made the change to save the revisionId if it is not being saved.

Tested the code with the following commands.

$ ./run_tests -v

Bug: https://crbug.com/gerrit/13709
Bug: https://crbug.com/gerrit/13707


Tested-by: Raman Tenneti <rtenneti@google.com>
Change-Id: I95fdf655b19648ad3e9aba10b9bed8bb9439deb6
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/296182


Reviewed-by: Ian Kasprzak <iankaz@google.com>