R1:5741514aeb5b

When a client submits an overlong "sourcePath", truncate it and continue

Summary:
Ref T13385. Currently, if you run `arc diff` in a CWD with more than 255 characters, the workflow fatals against the length of the `sourcePath` database column.

In the long term, removing this property is likely desirable.

For now, truncate long values and continue. This only meaningfully impacts relatively obscure interactive SVN workflows negatively, and even there, "some arc commands are glitchy in very long working…