git_command: switch process capturing over to subprocess

Now that these code paths are all synchronous, there's no need to run
our own poll loop to read & pass thru/save output.  Delete all of that
and just let the subprocess module take care of it all.

Change-Id: Ic27fe71b6f964905cf280ce2b183bb7ee46f4a0d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297422


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Reviewed-by: Jonathan Nieder <jrn@google.com>
Tested-by: Mike Frysinger <vapier@google.com>