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>

manifest_xml.py

@@ -31,7 +31,7 @@ else:
   urllib.parse = urlparse
 
 import gitc_utils
-from git_config import GitConfig
+from git_config import GitConfig, IsId
 from git_refs import R_HEADS, HEAD
 import platform_utils
 from project import RemoteSpec, Project, MetaProject
@@ -705,6 +705,10 @@ https://gerrit.googlesource.com/git-repo/+/HEAD/docs/manifest-format.md
             p.groups.extend(groups)
           if revision:
             p.revisionExpr = revision
+            if IsId(revision):
+              p.revisionId = revision
+            else:
+              p.revisionId = None
           if remote:
             p.remote = remote.ToRemoteSpec(name)
       if node.nodeName == 'repo-hooks':