The smudge/clean filter API expect an external process is spawned
to filter the contents for each path that has a filter defined.  A
new type of "process" filter API has been added to allow the first
request to run the filter for a path to spawn a single process, and
all filtering need is served by this single process for multiple
paths, reducing the process creation overhead.

* ls/filter-process:
  contrib/long-running-filter: add long running filter example
  convert: add filter.<driver>.process option
  convert: prepare filter.<driver>.process option
  convert: make apply_filter() adhere to standard Git error handling
  pkt-line: add functions to read/write flush terminated packet streams
  pkt-line: add packet_write_gently()
  pkt-line: add packet_flush_gently()
  pkt-line: add packet_write_fmt_gently()
  pkt-line: extract set_packet_header()
  pkt-line: rename packet_write() to packet_write_fmt()
  run-command: add clean_on_exit_handler
  run-command: move check_pipe() from write_or_die to run_command
  convert: modernize tests
  convert: quote filter names in error messages