In RawConfigParser:
  # ';' is a comment delimiter only if it follows
  # a spacing character
and this applies only on the first line of the value.

By escaping the first newline, which is by the way useless and ugly,
a ';' on the second line (i.e. the first continuation line) would be saved
on the first line in .installed.cfg.

This resulted in parts being reinstalled without reason.

WARNING: Newer versions of buildout don't use RawConfigParser anymore so it
         may not be affected by this bug and in any case, this commit must not
         be cherry-picked blindly.