upload/editor: fix bytes/string confusion

The upload module tries to turn the strings into bytes before passing
to EditString, but it combines bytes & strings causing an error.  The
return value might be bytes or string, but the caller only expects a
string.  Lets simplify this by sticking to strings everywhere and have
EditString take care of converting to/from bytes when reading/writing
the underlying files.  This also avoids possible locale confusion when
reading the file by forcing UTF-8 everywhere.

Bug: https://crbug.com/gerrit/11929
Change-Id: I07b146170c5e8b5b0500a2c79e4213cd12140a96
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/245621


Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>