sync: Add --superproject-rev flag to sync to specific revision

Allow syncing the outer manifest to a state defined by a specific
superproject revision. It updates the superproject, reads the manifest
commit from .supermanifest, and checks out the outer manifest project
to that commit.

Submanifests are then processed normally, allowing them to be updated
to the revisions specified in the new outer manifest state.

Bug: 416589884
Change-Id: I304c37a2b8794f9b74cb7e5e209a8a93762bdb52
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/576321
Commit-Queue: Gavin Mak <gavinmak@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

man/repo-smartsync.1

@@ -1,5 +1,5 @@
 .\" DO NOT MODIFY THIS FILE!  It was generated by help2man.
-.TH REPO "1" "August 2025" "repo smartsync" "Repo Manual"
+.TH REPO "1" "May 2026" "repo smartsync" "Repo Manual"
 .SH NAME
 repo \- repo smartsync - manual page for repo smartsync
 .SH SYNOPSIS
@@ -101,6 +101,10 @@ implies \fB\-c\fR
 \fB\-\-no\-use\-superproject\fR
 disable use of manifest superprojects
 .TP
+\fB\-\-superproject\-revision\fR=\fI\,SUPERPROJECT_REVISION\/\fR
+sync to superproject revision (applies to outer
+manifest)
+.TP
 \fB\-\-tags\fR
 fetch tags
 .TP

man/repo-sync.1

@@ -1,5 +1,5 @@
 .\" DO NOT MODIFY THIS FILE!  It was generated by help2man.
-.TH REPO "1" "August 2025" "repo sync" "Repo Manual"
+.TH REPO "1" "May 2026" "repo sync" "Repo Manual"
 .SH NAME
 repo \- repo sync - manual page for repo sync
 .SH SYNOPSIS
@@ -101,6 +101,10 @@ implies \fB\-c\fR
 \fB\-\-no\-use\-superproject\fR
 disable use of manifest superprojects
 .TP
+\fB\-\-superproject\-revision\fR=\fI\,SUPERPROJECT_REVISION\/\fR
+sync to superproject revision (applies to outer
+manifest)
+.TP
 \fB\-\-tags\fR
 fetch tags
 .TP