STYLE: Add detail to the vxl update instructions

Make the working directory for each command sequence explicit.  State
the purpose of the script recorded in the upstream snapshot branch
commit messages.  Add ascii-art to show the history shape as it is
constructed.  Add a Change-Id line to the upstream snapshot commits so
that the entire update topic can be reviewed in Gerrit.

Change-Id: I849330fe6bdd5c48f70c2f0a7916328936b3e9de
Merge topic 'vxl-update-instructions'

6f39a08 STYLE: Add detail to the vxl update instructions