repo: decode/encode all the subprocess streams

We use subprocess a lot in the wrapper, but we don't always read
or write the streams directly.  When we do, make sure we convert
to/from bytes before trying to use the content.

Change-Id: I318bcc8e7427998348e359f60c3b49e151ffbdae
Reported-by: Michael Scott <mike@foundries.io>
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/236612
Reviewed-by: Michael Scott <mike@foundries.io>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Michael Scott <mike@foundries.io>