project: fix encoding handling with git commands

The GitCommand Wait helper takes care of decoding bytes to strings
for us.  That means we don't have to decode stdout ourselves which
is what our local rev list, ls-remote, and generic get_attr helpers
were doing.

If we don't use Wait though to capture the output but instead go
directly to the subprocess stdout, we do have to handle decoding
ourselves.  This is what the diff helpers were doing.

Bug: https://crbug.com/gerrit/10418
Change-Id: I057ca245af3ff18d6b4a074e3900887f06a5617d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/233076
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>