Add 'repo sync -n' to only do the network transfer

This makes it easier to update all repositories, without actually
impacting the working directory, or learning about how to use
`repo forall -c 'git fetch $REPO_REMOTE' `.

Signed-off-by: Shawn O. Pearce <sop@google.com>

subcmds/sync.py

@@ -47,6 +47,9 @@ the manifest.
 """
 
   def _Options(self, p):
+    p.add_option('-n','--network-only',
+                 dest='network_only', action='store_true',
+                 help="fetch only, don't update working tree")
     p.add_option('--no-repo-verify',
                  dest='no_repo_verify', action='store_true',
                  help='do not verify repo source code')
@@ -90,6 +93,10 @@ the manifest.
       else:
         print >>sys.stderr, 'warning: Skipped upgrade to unverified version'
 
+    if opt.network_only:
+      # bail out now; the rest touches the working tree
+      return
+
     if mp.HasChanges:
       if not mp.Sync_LocalHalf():
         sys.exit(1)