git-gui: replace encoding binary with iso8859-1

git-gui: replace encoding binary with iso8859-1

git-gui currently configures some channels as '-encoding binary' when
the channel is not really binary (e.g, the channel is consumed as lines
of text).  In 8.6, '-encoding binary' is an alias for '-encoding
iso8859), but TIP 699 removes this alias for Tcl 9.0. Let's switch to
'-encoding iso8859-1' to be compatible across Tcl versions.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>