git-gui: assure -eofchar {} on all channels

git-gui: assure -eofchar {} on all channels

Per 6eb420ef61 ("git-gui: Always disable the Tcl EOF character when
reading", 2007-07-17), git-gui should disable Tcl's EOF character
detection on all files when on Windows: the default is disabled on all
other platforms (and with Tcl 9.0, is disabled on Windows too).  This
EOF character is for compatibility with files / applications written for
file systems that know only the disc sectors allocated, and not the
number of bytes used.  This has nothing to do with git.

But, git-gui does not set -eofchar {} on all channels.  To avoid any
further leakage, let's just add this to the Windows specific override of
open.  This override is needed only as long as Tcl 8.x is in use (Tcl
9.0 makes -eofchar {} default on all platforms).

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