sync: Support SYNC_BRANCH to override the smart sync branch

`repo sync -s` always sends the manifest branch to the manifest server
as the `branch` argument of GetApprovedManifest. Some servers map one
manifest branch to several server-side (CI) branches, and the only way
to pick among them was indirectly through the target.

Honor a SYNC_BRANCH environment variable, mirroring SYNC_TARGET: if set
and non-empty, send it instead of the manifest branch. An empty value
is treated as unset. -t/--smart-tag is unaffected.

Bug: 568962967
Test: ./run_tests
Change-Id: I26700e8997c21487f83011258100fd91a5c87508
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/639123
Commit-Queue: Rahul Yadav <yadavrah@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Tested-by: Rahul Yadav <yadavrah@google.com>

subcmds/sync.py

@@ -2059,7 +2059,12 @@ later is required to fix a server side protocol bug.
 
         return server_url, transport
 
-    def _SmartSyncSetup(self, opt, smart_sync_manifest_path, manifest):
+    def _SmartSyncSetup(
+        self,
+        opt: optparse.Values,
+        smart_sync_manifest_path: str,
+        manifest: XmlManifest,
+    ) -> str:
         if not manifest.manifest_server:
             raise SmartSyncError(
                 "error: cannot smart sync: no manifest server defined in "
@@ -2077,7 +2082,12 @@ later is required to fix a server side protocol bug.
         try:
             server = xmlrpc.client.Server(server_url, transport=transport)
             if opt.smart_sync:
-                branch = self._GetBranch(manifest.manifestProject)
+                # SYNC_BRANCH overrides the branch sent to the manifest server
+                # (e.g. to select a specific CI branch that builds the same
+                # manifest branch). An empty value is treated as unset.
+                branch = os.environ.get("SYNC_BRANCH")
+                if not branch:
+                    branch = self._GetBranch(manifest.manifestProject)
                 target = None
                 if "SYNC_TARGET" in os.environ:
                     target = os.environ["SYNC_TARGET"]