When exclude patterns are set, JujutsuClient diffs each changed file

separately with jj diff -- <filename>. jj parses these arguments as

fileset expressions, not literal paths. A name containing characters

like ,, (, |, or ~ then fails to parse, and the whole diff fails

with a generic error. For example, this happened for the test data file

CVSROOT/checkoutlist,v.
Each name is now passed as an exact file:"..." pattern, with quotes

and backslashes escaped.

Ran unit tests.
Ran rbt diff 'root()' @ with EXCLUDE_PATTERNS set. It had

  failed before and now produces the full diff.