io, dif: #519: FIX DIFF freeze when reading from GIL

+ CAUSE: In Windows, Diffs freeze while reading Popen streams,
probably buffers smaller; good-thin(TM) in this case because reading a
Popen-proc from the launching-thread freezes GIL.  The alternative to
use `proc.communicate()` also relies on big buffers.
+ SOLUTION: Use `cmd.handle_process_output()` to consume Diff-proc
streams.
+ Retroffited `handle_process_output()` code to support also
byte-streams, both Threading(Windows) and Select/Poll (Posix) paths
updated.

- TODO: Unfortunately, `Diff._index_from_patch_format()` still slurps
input; need to re-phrase header-regexes linewise to resolve it.

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