git_command: rework stdin handling

We only provide input to GitCommand in one place, so inline the logic
to be more synchronous and similar to subprocess.run.  This makes the
code simpler and easier to understand.

Change-Id: Ibe498fedf608774bae1f807fc301eb67841c468b
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297142


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