add -w option for |git bz attach| to ignore whitespace

Generating patches that ignore whitespace is occasionally helpful when
patches reindent large chunks of code.  Doing so helps the reviewer focus
on important code changes, rather than verifying that blocks of code were
copied appropriately.
add documentation for |git bz attach -w|
Merge pull request #17 from froydnj/ignore-whitespace

add -w option for |git bz attach| to ignore whitespace