Merged from trunk 5740:5746

Merged from trunk 5740:5746

Use XARGS for portable xargs handling when find doesn't find any files.

Minor fixes thanks to Fabio Correa <facorread@alexandria.cc> in bug
#162647.

Don't require MANIFEST2 hashes since that can trigger excessive fetches
when sufficient digests already exist.  Thanks to vapier for reporting.

To ease transition while Manifest 1 is being removed, only require
hashes that will exist before and after the transition.

svn path=/main/branches/prefix/; revision=5748