sub-process: use child_process.args instead of child_process.argv

sub-process: use child_process.args instead of child_process.argv

Currently the argv is only allocated on the stack, and then assigned to
process->argv.  When the start_subprocess function goes out of scope,
the local argv variable is eliminated from the stack, but the pointer is
still kept around in process->argv.

Much later when we try to access the same process->argv in
finish_command, this leads us to access a memory location that no longer
contains what we want.  As argv0 is only used for printing errors, this
is not easily noticed in normal git operations.  However when running
t0021-conversion.sh through valgrind, valgrind rightfully complains:

==21024== Invalid read of size 8
==21024==    at 0x2ACF64: finish_command (run-command.c:869)
==21024==    by 0x2D6B18: subprocess_exit_handler (sub-process.c:72)
==21024==    by 0x2AB41E: cleanup_children (run-command.c:45)
==21024==    by 0x2AB526: cleanup_children_on_exit (run-command.c:81)
==21024==    by 0x54AD487: __run_exit_handlers (in /usr/lib/libc-2.26.so)
==21024==    by 0x54AD4D9: exit (in /usr/lib/libc-2.26.so)
==21024==    by 0x11A9EF: handle_builtin (git.c:550)
==21024==    by 0x11ABCC: run_argv (git.c:602)
==21024==    by 0x11AD8E: cmd_main (git.c:679)
==21024==    by 0x1BF125: main (common-main.c:43)
==21024==  Address 0x1ffeffec00 is on thread 1's stack
==21024==  1504 bytes below stack pointer
==21024==

These days, the child_process structure has its own args array, and
the standard way to set up its argv[] is to use that one, instead of
assigning to process->argv to point at an array that is outside.
Use that facility automatically fixes this issue.

Reported-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>