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.

% test multiple merges at once
initializing destination source-hg repository
scanning source...
sorting...
converting...
4 Initial add
3 Added branch1 file
2 Added parent file
1 Added brach2 file
0 Merged branches
o    5 "(octopus merge fixup)" files:
|\
| o    4 "Merged branches" files: file-branch2
| |\
o---+  3 "Added brach2 file" files: file-branch2
 / /
| o  2 "Added parent file" files: file-parent
| |
o |  1 "Added branch1 file" files: file file-branch1
|/
o  0 "Initial add" files: file

% manifest of tip
644   file
644   file-branch1
644   file-branch2
644   file-parent