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 directories in SVN" is still better than "arc diff fatals".

Test Plan:

• Modified arc to submit very long source paths.
• Ran arc diff.
  • Before: Fatal when inserting >255 characters into sourcePath.
  • After: Path truncated at 255 bytes.

Maniphest Tasks: T13385

Differential Revision: https://secure.phabricator.com/D20727