numtide / treefmt / 25289813285

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walk: unblock producers when Close() is called before EOF

When the format loop exits early (Ctrl+C, or `on-unmatched = fatal`),
walker.Close() is called while producers are still blocked on send --
git/jj on a full 64 KB stdout pipe (~1.6k paths), the filesystem walker
on a full filesCh -- so errgroup.Wait() hangs until kill -9.

Have Close() fire a cancellation signal first: a context for the git/jj
readers (also kills the child via exec.CommandContext) and a done
channel that filepath.Walk selects against. Regression tests overflow
the buffer and assert Close() returns without Read() being called.

47 of 61 new or added lines in 3 files covered. (77.05%)

878 of 2117 relevant lines covered (41.47%)

17.18 hits per line