[RFC/PATCH 0/6] build argv_array into run-command

[Jeff King <peff@peff.net>]

The memory ownership of the "argv" array of a "struct child_process" can
be tricky. The child_process does not own the memory, but it must remain
valid until finish_command runs. That's easy for cases where we call
start_command and finish_command in the same function: you can use a
local array variable, or use an argv_array and cleanup afterwards.

But it's easy to screw up in cases where you want to start a command in
one function and finish it in another, either by pointing to invalid
storage during finish_command, or by leaking dynamically allocated
memory.

This series sticks an argv_array inside the "struct child_process",
which we clean up automatically.  Because some callers might not want to
use it, it's optional. If you provide "argv", we use that, and
otherwise fall back to the internal array.

The first commit below does that. The second fixes an uninitialized
memory access. 3, 4, and 5 plug memory leaks. 6 is just a cleanup for
consistency with the changes in 4 and 5.

And in 2, 3, and 5 we are introducing argv_array into new spots, which
simplifies the code and gets rid of magic numbers.

  [1/6]: run-command: store an optional argv_array
  [2/6]: run_column_filter: use argv_array
  [3/6]: git_connect: use argv_array
  [4/6]: get_helper: use run-command's internal argv_array
  [5/6]: get_exporter: use argv_array
  [6/6]: get_importer: use run-command's internal argv_array

-Peff