git-svn: update svn mergeinfo test suite

git-svn: update svn mergeinfo test suite

Add a partial branch (e.g., a branch from a project subdirectory) to the
git-svn mergeinfo test repository.

Add a tag and a branch from that tag to the git-svn mergeinfo test repository.

Update the test script to expect a known failure in git-svn exposed by these
additions where merge info for partial branches is not preserved.

Signed-off-by: Andrew Myrick <amyrick@apple.com>
Acked-by: Eric Wong <normalperson@yhbt.net>