sync: use manifest_name passed in

Commit fb527e3f

 ("sync: create dedicated
manifest project update func") refactored code from the main body into a
dedicated method.  The manifest_name was passed as an argument, but never
used it, and instead reaches back out to the command line options.  This
ignores the logic in the main loop where manifest_name might have changed
(like when using smart sync).

Change-Id: I4b84638fbb10c2b6f8f4b555e1475b0669c2daf4
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/305148
Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: Mike Frysinger <vapier@google.com>