manifest_xml: correct project revisionId for extend-project

Using sha1 manifest, project's revisionId is initialized
first by the manifest.
An update of a projet revision by extend-project node does
not apply to the revisionId which is therefore kept to the
initial value.

Resets revisionId value when revision is updated by an
extend-project node.

Change-Id: I873af283890cebaeaabde966f04b125642af929f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/275715
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Miguel Gaio <miguel.gaio@renault.com>