More warnings.
* svn-merge2git.sh (warn): record warnings with a timestamp in the
global $warn_msgs.
(create_graft): Count the number of grafts not created because they
already existed.
(rewrite_history): Print the commits of which we couldn't figure out
the merge parent before rewriting the history. Print more useful
warnings with --dry-run instead of short-circuiting the code too
early.
(doit): Record commits of which no merge parent could be found.
(main loop): Adjust accordingly.
(end of script): Print more summary information.
Signed-off-by: Benoit Sigoure <tsuna@lrde.epita.fr>