convert/gnuarch: parse continuation-of revisions in gnuarch source In GNU Arch, continuation-of was often used for: - tagging revisions - continue working on a project in a new archive, because arch was scaling poorly in revision numbers (cat-logs were slow to be parsed and scanned through) - very similar to the previous point, fork his own branch of a project. Parsing this header information will allow to 'follow' new history because it often hints at older/forked/personal revision trees. This patch however just implements the parsing of the continuation-of header. A followup patch will implement the proper use of this new information.

adding empty-file
adding large-file
adding another-file
removing empty-file
removing large-file
recording removal of large-file as rename to another-file (99% similar)
% comparing two empty files caused ZeroDivisionError in the past
2 files updated, 0 files merged, 1 files removed, 0 files unresolved
adding another-empty-file
removing empty-file
adding large-file
adding tiny-file
removing large-file
adding small-file
removing tiny-file
recording removal of tiny-file as rename to small-file (82% similar)
% should all fail
abort: similarity must be a number
abort: similarity must be between 0 and 100
abort: similarity must be between 0 and 100