git-svn: Make following parents atomic

git-svn: Make following parents atomic

find_parent_branch generates branch@rev type branches when one has to
look back through SVN history to properly get the history for a branch
copied from somewhere not already being tracked by git-svn.  If in the
process of fetching this history, git-svn is interrupted, then when one
fetches again, it will use whatever was last fetched as the parent
commit and fail to fetch any more history which it didn't get to before
being terminated.  This is especially troubling in that different
git-svn copies of the same SVN repository can end up with different
commit sha1s, incorrectly showing the history as divergent and
precluding easy collaboration using git push and fetch.

To fix this, when we initialise the Git::SVN object $gs to search for
and perhaps fetch history, we check if there are any commits in SVN in
the range between the current revision $gs is at, and the top revision
for which we were asked to fill history.  If there are commits we're
missing in that range, we continue the fetch from the current revision
to the top, properly getting all history before using it as the parent
for the branch we're trying to create.

Signed-off-by: Deskin Miller <deskinm@umich.edu>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>