lei p2q: fix /dev/null filenames, fix phrase quoting rules

lei p2q: fix /dev/null filenames, fix phrase quoting rules

/dev/null mis-handling was reported by Kyle Meyer.

Phrases quoting rules are also refined to avoid leaving spaces
unquoted when "phrase generator" characters exist.  Also,
context-free hunk headers no longer clobber the in_diff
state of the parser, since git can still generate those.

Link: https://public-inbox.org/meta/87k0qrrhve.fsf@kyleam.com/