Remember the previously stored base diff path.

Users who use the Upload Diff form instead of post-review against Subversion
repositories (and others) have to type a base diff path every time, which
can be annoying. This change stores the base diff path on uploads and re-uses
it as the default for the field the next time the user goes to upload a
diff.

Uploaded a diff with a base path. Next time I went to upload, I saw the previous base path.

Unit tests all pass.