git-svn: Fix for svn paths removed > log-window-size revisions ago

git-svn: Fix for svn paths removed > log-window-size revisions ago

Instead of trying to find the end of the commit history only in the
last window, track if we have seen commits yet, and use that to judge
if we need to backtrack and look for a tail.  Otherwise, conversion
can silently lose up to 100 revisions of a branch if it was deleted
>100 revisions ago.

Signed-off-by: Alex Vandiver <alexmv@mit.edu>
Acked-by: Eric Wong <normalperson@yhbt.net>