Fix an issue with validating against repos that don't use basedir.
Review Request #4268 — Created June 28, 2013 and submitted
Fix an issue with validating against repos that don't use basedir. The UploadDiffForm had a buried case where it set the basedir to the empty string in cases where the repository uses absolute paths, and it turns out that this is necessary because we pass the value to os.path.join(), which really doesn't like getting None for its first argument. I had missed this when implementing ValidateDiffResource, which meant it worked for svn but didn't for git.
Validated some diffs against a git repository.