credential-cache: use child_process.args

credential-cache: use child_process.args

As child_process structure has an embedded strvec args for
formulating the command line, let's use it instead of using
an out-of-line argv[] whose length needs to be maintained
correctly.

Also, when spawning a git subcommand, omit it from the command list
and instead use the .git_cmd bit in the child_process structure.

Signed-off-by: Junio C Hamano <gitster@pobox.com>