Merge branch 'jt/subprocess-handshake' into maint

Code cleanup.

* jt/subprocess-handshake:
  sub-process: refactor handshake to common function
  Documentation: migrate sub-process docs to header
  convert: add "status=delayed" to filter process protocol
  convert: refactor capabilities negotiation
  convert: move multiple file filter error handling to separate function
  convert: put the flags field before the flag itself for consistent style
  t0021: write "OUT <size>" only on success
  t0021: make debug log file name configurable
  t0021: keep filter log files on comparison

diff --cc cache.h
Simple merge