git_command: fix input passing

After reworking this function to use subprocess for output capturing
in commit c87c1863 ("git_command:
switch process capturing over to subprocess"), passing input via
stdin write no longer works.  We have to pass it via communicate(),
and we have to pass it a string instead of bytes (since we always
use encoding='utf-8' now).  This is fine since the only user of the
input= setting today is already passing in a string.

Bug: https://crbug.com/gerrit/16151
Change-Id: Ic58db1e568b8f8aa840a6d62c5a157c14aa6d9bc
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/343515


Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: LaMont Jones <lamontjones@google.com>