Merge branch 'ak/p4'

Merge branch 'ak/p4'

* ak/p4:
  Utilise our new p4_read_pipe and p4_write_pipe wrappers
  Add p4 read_pipe and write_pipe wrappers
  Put in the two other configuration elements found in the source
  Put some documentation in about the parameters that have been added
  Move git-p4.syncFromOrigin into a configuration parameters section
  Consistently use 'git-p4' for the configuration entries
  If the user has configured various parameters, use them.
  Switch to using 'p4_build_cmd'
  If we are in verbose mode, output what we are about to run (or return)
  Add a single command that will be used to construct the 'p4' command
  Utilise the new 'p4_system' function.
  Have a command that specifically invokes 'p4' (via system)
  Utilise the new 'p4_read_pipe_lines' command
  Create a specific version of the read_pipe_lines command for p4 invocations

Conflicts:
	contrib/fast-import/git-p4