Fix 'Bootstrap' to bypass a problem with "bk changes" by changing the directory before calling the command.


Build-tools/Bootstrap:
  Bypass a BK problem with "bk changes": It stopped delivering changeset comments when
  1) it encountered a single, unbroken line with about 1k characters  AND
  2) it was applied not to the current directory but to a named BK root directory.
  
  As 1) cannot be changed post-factum (and might happen again),
  the "bk changes" command is now preceded by a "cd $REPO".
  
  Do some renaming of variables used around the "bk changes" call to improve documentation.