assist: operate on all files in working tree by default

assist: operate on all files in working tree by default

Consistency with sync and internal consistency is more important than
consistency with the assistant, which is not itself consistent about
what it does when run in a subdirectory.

Note that with -C, it will still commit staged changes to files outside
the directory. Like sync does. Presumably if the user is manually
staging things, then running this command, they intend to build up a
commit.

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