repo: handle bad programs a bit better

If programs emit non-UTF-8 output, we currently throw a fatal error.
We largely only care about the exit status of programs, and even the
output we do parse is a bit minimal.  Lets make it into a warning and
mangle the invalid bytes into U+FFFD.  This should complain enough to
annoy but not to break when it's not necessary.

Bug: https://crbug.com/gerrit/12337#c2
Change-Id: Idbc94f19ff4d84d2e47e01960dd17d5b492d4a8a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255272
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>