git svn: handle SVN merges from revisions past the tip of the branch

git svn: handle SVN merges from revisions past the tip of the branch

When recording the revisions that it has merged, SVN sets the top
revision to be the latest revision in the repository, which is not
necessarily a revision on the branch that is being merged from.  When
it is not on the branch, git-svn fails to add the extra parent to
represent the merge because it relies on finding the commit on the
branch that corresponds to the top of the SVN merge range.

In order to correctly handle this case, we look for the maximum
revision less than or equal to the top of the SVN merge range that is
actually on the branch being merged from.

[ew: This includes the following (squashed) commit to prevent
     errors during bisect:]

  Author: Toby Allsopp <toby.allsopp@navman.co.nz>
  Date:   Fri Nov 13 09:48:39 2009 +1300

    git-svn: add (failing) test for SVN 1.5+ merge with intervening commit

    This test exposes a bug in git-svn's handling of SVN 1.5+ mergeinfo
    properties.  The problematic case is when there is some commit on an
    unrelated branch after the last commit on the merged-from branch.
    When SVN records the mergeinfo property, it records the latest
    revision in the whole repository, which, in the problematic case, is
    not on the branch it is merging from.

    To trigger the git-svn bug, we modify t9151 to include two SVN merges,
    the second of which has an intervening commit.  The SVN dump was
    generated using SVN 1.6.6 (on Debian squeeze amd64).

Signed-off-by: Toby Allsopp <toby.allsopp@navman.co.nz>
Acked-by: Eric Wong <normalperson@yhbt.net>