repo 1.6.2

Add repo manifest -o to save a manifest

This can be useful to create a new manifest from an existing client,
especially if the client wants to use the "-r" option to set each
project's revision to the current commit SHA-1, making a sort of a
tag file that can be used to recreate this exact state elsewhere.

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