bug: shell fallback for try-summary and and try-commit break on filenames with newlines

[angelhof]

By @mgree:

The logic could break with particularly nasty filenames (e.g., newlines). The most robust approach is to null terminate, à la find ... -print0 or xargs -0.

(See the discussion in #83)

[mgree]

I was going to take a whack at fixing this in the shell fallbacks in #158, but it's a bigger overhaul than I initially expected: we can't store the results of find -print0 in a variable. 🥲