graph-ancestry should escape commit message

 affects /products/bzrtools

graph-ancestry needs to escape backslashes in commit messages to prevent
dot syntax errors:

ganieda:~/bzr/bzr.dev% bzr graph-ancestry . foo.dot
ganieda:~/bzr/bzr.dev% dot -Tpng foo.dot -o foo.png
Error: foo.dot:5095: syntax error near line 5095
context: n2311[fillcolor="white" style="filled" label="62e3e\nAaron
Bentley\nwin32fixes\n2006/03/27\nd2202" shape="box" tooltip="Used
osutils getcwd instead of replacing \"\\" with >>> \ <<< "/\""
href="#"]